Smite is obvious, you are a jungler after all (i dont really like heca on top)

Ghost- Hecarim benefits from this rune very much, bonus ms is good both for escaping and chasing. You usualy want to pop this at the start of the fight.

Flash- Also i really fine option. True, Hecarim Maybe benefits more from ghost, but this gives you a really nice gap-closer, you will have plenty of ms no matter what, and its easier to use than Ghost, so if you really don't want ghost you can take flash, but keep in mind that ghost is a better option.

-Your start is obvious, but you take Refillable Potion instead of 3 normal hp pots because its more gold efficient.

-First back: Boots will give you what you need; ms (so some ad as well) and you will have your predator, Long Sword for more ad and Control Ward you should have whenever you have an empty slot and 75gold, at 1 point when you feel like you dont need refillable potion anymore, sell it for a control ward if you arent buying anything else.
-Second back: Ninja Tabi against heavy ad comps, Mercury's Treads agains ap comps, and Boots of Swiftness if the enemy team has some slows, and none of them got any early kills (more than 1).

-Rush: Why rush Trinity Force? Because it gives you everything you want; ms, ad, mana, hp, its passive really "clicks" with Hecarim's kit (spamable q mostly), + other stats it gives are also very useful bonus.

-Core: Youmuu's Ghostblade: gives you lots of lots of ad, lethality is nice, and it's passive is great with hecarim's passive and Celerity (cdr is also nice)
Dead Man's Plate: Gives you tankiness, and a lot of burst from its passive.
Turbo Chemtank: Again, tankiness, its active gives synergies greatly with your passive and celerity, cdr, and also gives you mana.

-Adapting: Randuin's Omen is a great item because (besides giving you tankiness) of its passives, and also its active is nice, and as we are using Ingenious Hunter, you'll get to use it a lot. You want to take this item when enemy comp is mostly ad, and their magic dmg dealers aren't threatening you.
Maw of Malmortius and Mercurial Scimitar: So yeah, you want to take 1 of these if enemy ap-s are a threat. Scimitar if their team has a lot of cc, Maw if you aren't taking Scimitar. Mercurial Scimitar also goes well with Ingenious Hunter, and it also synergies with your passive and Celerity, but that ms buff is short (whole 1sec), but as that ms buff is short, you wont feel it that much. (at least I don't)

-Other tanky items: If a game is rough, you should skip Dead Man's Plate, and buy 1 of these: Thornmail-dont forget that it also aplies grevious-wunds, its really good into Warwick, Master Yi, etc... or Sterak's Gage I love this item, and if you dont get too ahead early (or even stay equal to the enemy jungler), you should take this instead of Dead Man's Plate, and also, always when youfinish your build, if you bought dead man's plate, sell it for sterak's gage.

-If your tean doesn't have a tank, don't take Youmuu's Ghostblade, take Dead Man's Plate after Trinity Force, than one item from "adepting" items I listed and than take sterak's, and if their team comp's ap-s are really threatening, take another mr item from "adapting items i listed", and if your team has some magic dmg dealers, you can take Abyssal Mask, if not, finish with taking righteus glory and randuin's omen.

-Where's the jungling item?? I dislike finishing jungling item on hecarim. I sell Hunter's Talisman later on, (but you want to keep it as long as possible cause of the bonus exp, and mana). Why? Because that is what works for me, what makes hecarim feel smooth for me.

-Why take Elixir of Iron over Elixir of Wrath? Well, you wont always take elixir of iron, if your team is hard winning, take elixir of wrath and look to end the game as fast as possible. If you are even, or behind, (or your team doesn't have a tank) take elixir of iron.

I'm preatty sure that you know how his abilities look like, so im just going to go over them quickly.

PASSIVE( Warpath) - Hecarim gains Attack Damage equal to a percentage of his bonus Movement Speed.

Q ( Rampage)- you deal dmg in a relatively short range around you, has a 4 sec cd without cdr, when you hit something its base cd is lowerd by 1 sec for a short period of time, and that can stack 2 times. (phisical dmg, ad scaling)

W ( Spirit of Dread)- You deal dmg per second around you for 4s, it also heals you, but heal recived from minions is capped. (magic damage, ap scaling)

E ( Devastating Charge)- This lasts for 4s, while it is active, you can go through units, your ms is increased, and further increases during first 3s, more you travell, more you will nock your target back, more dmg the ability it-self will do, and more dmg because of your passive you will do. (phisical dmg, ad scaling)

R ( Onslaught of Shadows)-Hecarim summons spectral riders and charges forward, dealing magic damage in a line. Hecarim creates a shockwave when he finishes his charge, causing nearby enemies to flee in terror. (magic dmg, ap scaling).

Now hecarim is a pretty simple champion, you spam your q when en enemy unit/wave or a monster is near, your W as well. You use your E to get faster to your destination, and add a bit of dmg while clearing the jungle. If you ganked and you are already behind your enemy, but your E isn't isnt at max speed, I would wait for a moment to get it to its max speed, as than it does more dmg and is a bigger nock-back.
-Your lets say burst combo wouldev been this-> E (fully "charged") --> (while hitting the target with your charge) Q and W. From this point you use yout aa-s and Q. To this you can add predator, but keep in mind that if you are cheneling predator and you use your E, that will cancel the chanel, so you should use your predator before anything else, while you are still getting to the fight for e.g., or just wait in the bush untill the chanel is over, than go and make somebody's day with your burst and speed. (needless to say, don't forget to use your other item's actives)

Start with q ( Rampage)-most dmg at the start, W ( Spirit of Dread) second because it gives you better jungle clear than your E ( Devastating Charge), and it will also heal you, and than you take e. You wanna max Q because of your jungle clear and because it will its your main dmg source after you hit your E (q-->aa-->q... ofcourse you want to use your w as well). Than you max your E because that ability is source of your burst, and because your w isn't that useful. Of course, you take your ult whenever you can.

Predator - Simply great. Adds tons of ms which helps you get behind your enemies
easier before your Devastating Charge expires, ads burst by its self
and it is really fun to use! (ofcourse there is the synergy with Celerityand your passive as well)
Sudden Impact - gives you lttle more burst with your E as well.
Eyeball Collection-more ad.
Ingenious Hunter - your active item abilities are a big part of your build. With them, when you activate them all (your boots too) you will 1-shot squshies, and you want those abilities to be up all the time!
Celerity - This is just to good, gives ms, and is also a weaker hecarim passive, perfect for us!
Waterwalking - This is really strong on Hecarim because it scales with your passive, with Celerity and gives ad on it's own, I know its only in the river, but as scuttles became more important, this gives you a better chance to contest them.

I want you to know, about match up difficulty, this is from my experience, someone will have a different opinion!

-As i already listed a lot of match ups in the "vs champions", and what should we do when we face those champions, here I'll list who should you ban and why.
-Bans: Shaco, Nocturne, Jax, Warwick, Elise

-Why should you ban these champions: Lets start with Jax and Warwick; you can't contest early scuttles which is terrible, you shouldn't be able to 1-shot them, and if you are just slightly ahead, you won't be able to duel them.
-Elise: She is though, but if you can dodge her stun it should be fine when you get to your trinity, but she can dodge your E, can stun you, and has 6 abilities at the point you have 3, and when you get your ult, if she is good, she can dodge that as well.
- Shaco: You can't do much against an experienced and good shaco player as Hecarim.
- Nocturne: Really strong right now, great duelist, so your chances of being able to 1v1 at any point of the game are thin, so you can't contest scuttles, and his ult makes his ganks easier to pull of. He will also almost match your map presence.

Pros: -Ok early -Good mid and late -Nice map presence -very good clear speed -(in low elo)amazing ganks -(in high elo)fine ganks -Really good dmg and burst -Really good chase -Really easy to escape (in most cases) -really fun to play -little bit tanky -good snowball -really adaptable

-Cons: -Not as tanky as some other hecarim builds (I can't think of anything else x) )

If you are on the blue team: red brambleback --> raptors --> scuttle --> blue --> gromp --> wolves --> go back --> krugs --> raptors again.

If you are on the red team: Blue --> gromp --> wolves --> red brambleback --> scuttle (if you find it, be careful while takeing it) --> look for a gank/ raptors --> gank/ go back.

Do not forget to kite the buffs, raptors, krugs, and wolves!

!Again, this is the clear that suits me the best, someone might have a slightly different opinion!

Well, at the start, follow the clear I gave you-it's in my opinion the best clear in the sense of gold, as it will give you an ideal first back. Be on the look out for ganks, as your ganks are really strong and you snowball really hard. If you got a few kills, and you are ahead of the enemy junglers, if you think that enemy won't escape if you leave the kill fotr your teamm8, as that way you will set them far ahead of their lane oponent, and give them a better chance to carry with you. If you get ahead, always look for pick-offs, suiciding for 1 kill isnt worth if you are ahead, and force objectives, and end the game as fast as possible. If you fall behind, try to make some good calls, try split-pushing if you see a good opertunity and try picking enemies off, even if your team is behind, you can win fights with numbers, force a, lets say, 4v2, that you should win, giving you much needed gold, and an opertunity to take a tower or drake- those plays will put you back on the horse (:D), "catch the game they threw!"
some tips:
1. I mentioned split-pushing, while doing that, make sure you warded well, don't be greedy, when you see multiple oponents coming top, ult over a wall without hesetation, and use your E to get away as quickly as possible. (if you dont have ult, pick what you think is the safest route, and run that way).
2. Always keep your eyes on the lanes, now this is for other junglers as well, but you never know when an opertunity for a perfect gank will come.
3. At the start of the game look at oponents bot and top, if for e.g. top came late to lane, that almost for sure means that enemy jungler started at the top side. Doing this will give you a nice advantage over your oponent, as you will know better where he is, than he/she will know where you are.
4. Once you get ahead, never leave them a chance to get back into the game-constantly gank, take objectives, pick them off, that is how you win the game, if your pressure disapears just for a brief moment, that will leave a gap which your oponents can use to gat something they need-objectives/gold. -Basicly close out games as fast as possible.