Hecarim is one of those champions that you either do really good in a game or you suck absolute balls. There is no middle ground for Hecarim. Especially with his items. You either have to build damage or tank depending on your team. Do you have a Garen and Braum? Then go damage, your team has enough tanks.
You can try to build a bruiser build on Hec but it just won't work. For example, let's say your build is half damage and half tank, you won't be able to tank as well as you would think and then you won't do as much damage as you should. You literally become useless.
It's okay if you build maybe one armor/mr item in your damage build but that's it. The same goes for taking a damage item in a tank build. You don't want to be all tank and no damage.

Smite: It's pretty obvious.

Ghost: More Speed for more damage and with Distortion boots, even more speed

Flash: Always good for escapes, initiations, stealing drag, and everything else

TP: If you are not a high enough level or just took it by mistake, GET HOMEGUARDS. Make sure you have wards and go back to base. Proc HOMEGUARDS, TP to enemies and E them. You get a huge speed boost and enough damage to make for any early game feeding.

Start BLUE. That's all there is too it. By getting blue first you get cooldown on your Q and, when you level up, W. The way to go to clear the most of your jungle is: BLUE -> WOLVES -> CHICKENS -> RED (and if you have at least half health)-> KRUGGS

After CHICKENS you should have your E. Wait until you have all your skills off cooldown then back up, E, Tackle RED, W, Q. Smite RED when he's low to get the most of the heal.

Now Jungling Items:

You get CHALLENGING SMITE so that when you gank, you smite and then E tackle them. CHALLENGING SMITE let's you smite a champion so they take TRUE DAMAGE for 6 seconds. That's enough time for you to E, Q and have a Fiora Lunge on them with W and E.

CHILLING SMITE works to slow down enemies enough so that you and your teammate can jump them. That's about it. It's like a Malphite Q. It's good to secure a kill.

Although I prefer safe ganking by waiting until 6, you can gank at 3 if you are sure that your teammate will get the kill.
You also wanna make sure you gank the champions who get ridiculously broken (Katarina,Riven,LeBlanc,Olaf) if fed or if they are laning against someone like that.

Do NOT gank if the enemy champ is already fed. Gank another lane so that they can roam and help the lane that's struggling.

After the Jungle Item and Tier One Boots, get SHEEN. SHEEN is your go to item all the time. It provides you with enough damage for a gank early game. After that build Trinity Force or Iceborn Gauntlet.

Damage:
Trinity is your sole core item. Nothing should come before it other than your jungle item. Build tier 2 Boots then get Youmuu's for Armor Pen + the Speed Boost. Now you just build whatever damage you need.

Tank:
Iceborn is what you need for tanking. You are a tank so the next should be either your Sunfire Cape or a Frozen Heart. Then MR and next more Health with either Randuin's Omen or Righteous Glory.

The reason you NEVER EVER EVER take Mobility Boots is because the don't give you what you think it gives. Mobis give you a temporary speed boost that only works out-of-battle. Sure it might work when you charge in but that's it. With Boots of Swiftness you gain a permanent speed boost and Slow Resistance. And for Tanks, Mercury Treads provides MR and Tenacity. If you really want Movespeed, get a Zephyr. It gives 10% movespeed & CDR 50% Attack Speed and 25 Attack Damage.

So Hecarim is a hybrid champ with his W and ult needing AP. So if you are ahead or just messing around, get a Zhonya's. Zhonya's will let you ult in a teamfight, due a good amount of damage, GOLD HORSE.....then you W, Q, and E/Flash out. Do not think this is a good item though to build in ranked.