Flash- a must for Thresh top, you lack escape in your kit.
Ignite-so here is where you can change your pick, ignite is 1 option, It is good if you are a good thresh player (a very good thresh player), but still, if you are taking this into a ranked game (which I got a lot of reports for x) ), Exhaust and Teleport are definetly better options.
Exhaust- again, you lack escape, so an aditional slow and dmg debuff are very useful + it helps your jungler as well when he comes, there are a couple metch ups in which you need to take this, but i'll get to that.
Teleport-This is the best choice for a second summoner spell, unless your enemy top-laner's champion doesn't force you to take exhaust. Teleport gives you so much; you will have great opertunitys with it to tp-gank, and with those ganks you will shine: you will bring both dmg and cc to the fight; if you screw up it will give you a free back; If you roam by foot, this will help you defend your tower if needed.

Starting items- Long Sword I do prefer over Doran's Blade because even if doran's gives you that hp and 3% life steal, you get only 1 hp pot with it, so even though its stats, you will sustain longer with high hp with 3 pots.

First back- Even if getting B. F. Sword on your first back is ideal, I know it is a bit expensive, getting a Pickaxe is completely fine. Getting a Refillable Potion is great, but you dont need to take it, but you should take at least 1 Health Potion. Taking a Control Ward is a must, i doubt you will survive a good gank. If you really get screwed, getting another Long Sword, a control ward, and 1 hp pot is ok.

Statikk Shiv- it doesn't metter what you got on your first back, you are going to go for this next- it gives you attack speed which you desperetly need, crit chance is nice, it ads burst what you also need, I know that Rapid Firecannon helps out with your range, but the reason I prefer statikk shiv is that it can crit.

Core- After Statikk Shiv, best choice is Stormrazor as it will give you, again, everything you need - attack speed, ad and burst. Now, after buying stormtazor, think about your position in the game, are you ahead? If you are equal with your oponent, take Edge of Night - it will help you to deal dmg to a tanky oponent with it's lethality (ofcourse with its ad as well), and because of a bit of tankines this item gives, it will be harder for your oponent to score an advantage. If you are ahead, take Blade of the Ruined King-this item is will help you snowball so much, it gives you perfect stats for what we are trying to build here :). If you are behind, best option would be to go for a Frozen Mallet- it gives you ad, so your dmg won't fall off that much, and it gives you a lot of health, so you can survive a bit longer +it's passive will allow you to kite more easily.

Agressive items: Guinsoo's Rageblade- really powerfull item, it gives you nice stats, and its passives are really strong. Now I'll be honest, i don't know does this item completely work on Thresh, i mean, idk does it double its passive E dmg every other attack (For those who didn't know, thresh's E adds dmg without waiting between attacks).
Wit's End- you usualy wont be buying this item, you want to take it when you are against a tank/ tanky champion that built a lot of magic resist (your e on-hit effect deals magic dmg), so this item helps you to deal with their mr.
Bloodthirster- a lot of ad and life-steal, and it's passive shield is also helpful. Infinity Edge- This item is strong, but not always the best option, I like leaving it for the last item, and than selling Stormrazor and taking Phantom Dancer.

Defensive: Frozen Mallet- I already mentioned this item, gives ok ad, a lot of hp, and helps in kiting and chasing with its on-hit slow passive. Edge of Night- saying again, fine ad, lethality is really nice, bit of tankiness, and its active is really nice. Maw of Malmortius and Mercurial Scimitar are both really nice items. You should take at least one of them when facing an ap champion, or if enemy team is mostly made of magic dmg dealers. Now if you are against a champion that deals mostly magic dmg in lane, and enemy team has more ap champions, you can skip Blade of the Ruined King by taking Mercurial Scimitar, after which taking Wit's End, that will give you enough mr, while giving you a lot of dmg output.

Boot upgrade- Ninja Tabi against an heavy ad comp. Mercury's Treads against heavy ap comps. Berserker's Greaves- if you are ahead, but against a tank in lane, and at least 1 more tanky champion, though even than I would recomend tabi or treads, only when I would recomen greaves is when you are only ad in the team. Sorcerer's Shoes, you can take them when your team is full ad, remember, your E on-hit effect deals magic dmg.

Control Ward- Vision is really important, especily now when you are playing a champion with no mobility and no significant tankiness top.

Q- this is your displacement tool, engage tool, escape tool, and kill tool at the start (your way of getting solo kills at the early laning faze is draging your oponent into your tower range, even if he doesnt take the tower aggro, you will get a lot of free dmg, if he does take tower aggro on lvl 2/3 he is dead.). Your Q also makes you harder to dive, as when your oponent trys to dive you alone, you can just hook him, drag him aroun for a bit, and that E him avay from you, finishing off whith aa-s - And this ability has a long couldown, it is really inportant to land most of your hooks. And if you are trying to engage ( you are planing to use your second part of q-that dash), if there are minions next to your target, try to aim that way that if you miss them, you can hit the minion, and that dash to it (him or her? lets just say it).
W-you can throw it into a river/tri bush while being chased so that your oponent thinks that your jungler is here, they usualy back off - bonus effect of doing this is that when your jungler really comes, they might not back off when they see the lamp. I wouldn't advise you to use it as a shield, as shilding of this ability isn't that good, but if you are dying to ignite, it's worth a try.
E- Active effect of this ability is both an engage tool, and disengage tool. Passive part is your main dmg scource- its that on hit effect.
R- To help you lock down your targets, and to help you escape.

Start with E ( Flay) as i will help you poke and farm, also it won't allow your oponent to jump on you so easily. Than Q ( Death Sentence) as it is your main kill tool at the start (drag them into a turret). Even if you are alone in lane, take your W ( Dark Passage) third as it will give you the possibility to trick your oponent, and to pull off ganks which your target probably wont survive even with burning flash. Its best to max E as it is your main dmg tool. Than after that max Q and finish of with W. Ofcourse you put a point in your ult ( The Box) whenever you can.

Dark Harvest- helps out with your burst quite a lot, and feels so good while using it. True, taking another tree might be better early, but early you will be s++t any ways, and this helps you scale hard into the game. (ignore what i sad about being s++t early, you will be fine ;) ) And also this rune helps you poke in lane.

Taste of Blood- You really need extra sustain, and this gives you excetly that. But if you are hard smurfing, taking Sudden Impact can be fun.

Zombie Ward- your chances of surviving a good gank are slim, so you need this extra vision in case you fail in freezing your wave.

Relentless Hunter- good for roaming and getting back to your lane, but Ravenous Hunter is just as good-it will give you sustain and kinda some life-steal because of your E on hit effect.

Legend: Alacrity- gives you attack speed, and that is exectly what you need.

Coup de Grace-really nice rune for thresh, if you menage to hook your enemy top-laner, he will take atleast 2 tower shots (if you hook him/her with q and that e), at this point their hp will be close or even under 40%hp, so this rune helps you finish them off + later one, if they have a traditional adc, after first hit, they should be close or under (depends how/if you are fed), so again, this rune will kick in quickly.

So, at the start of the game, you will probably be weaker than your oponent, so you have to be smarter (just hug your tower). Frezzing the wave just out side your tower range is esential, but dont worry, that aint hard in low elo. When you freeze the wave, just look for the hook opertunitys, you got no idea hom many first-bloods I got with thresh by abusing the tower (especily back when i used to main thresh mid, good old times ;) ). Now, you should farm as you are an adc, your power spikes are big, but not cheap.

At the start of the game you should look at enemy team's 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 his/her jungler is, than he/she will know where yours is.

Team-fights- Be really carefull, as you are not tanky, if you engage with your q dash part, use R and E throw a your W (to your team) and get the hell out of front line (you can sneak 1 aa before you do so). When the fight actualy starts, just aa, and than walk around waiting for your Stormrazor to charge back up (if you find yourself having 100%crit chance, than just aa constantly) ofcourse you wanna aa the squishies first. When only the more tanky members of the enemy team remain, than start auto-attacking constantly. And use your cc as much as possible, take good opertunitys in fights.

Killing somebody- Aurelion Sol has a realy good line to describe this: "BOOP"
thats basicly how you kill somebody, you give them a big BOOP with your aa