Hello Summoners, I am GMagician, a League of Legends Support Main in Gold! The highest I've ever been is platinum 4, but I live in gold, for now! I'm going to keep this guide short and simple , because in the end it all comes down to how good you are at taking peoples souls , and no guide can help make you better at that (sorry for your loss lucian).

Because you got autofilled to support and you panic picked Thresh. Thresh is a great champion! He has a long range hook which brings you to the enemy team (easy suicide) he has this flay ability which can bring deadly assassins closer to your adc (no friendly fire doesn't mean you can't get your adc killed) he has a lantern to bring your adc with you into the enemy fountain (suicide pact?). Basically, if your adc is pissing you off and not thanking you for being the only reason he's doing good you can get him killed :) or yourself if that'll piss him off more. Your adc should respect you and if he doesn't he should die.

Passive - Damnation, you collect souls, you gain armor and ap as you collect souls, however you dont gain armor per level. You collect souls normally, just walkin around doin what you do so don't worry too much.
Q - Death Sentence, you throw your death reaper scythe (which takes 0.5 seconds) to sentence an enemy to death! Its a skill shot, and if you somehow land it you will pull the enemy a little bit, and you can press q again to go towards the enemy.
W - Dark Passage, you throw a lantern, and if someone grabs onto it for dear life (an ally) you will pull the lantern right back to you! Its almost as if your a friendly death reaper
E - Flay, probably your most useful ability. Theres a passive that charges up while your not aa'ing so your next aa goes BOOM and POW and deals more damage. If you ever wanna go full lethality + damage this bad boi will help you hit 50% of a carry if your fed :), it also lets you pull or push enemies so really good to peel or stop people from running
R - The Box, this is sad. You create a prison of walls, essentially a cage. Anyone who hits a wall gets slowed and hurt. People are stuck with the reaper.

Early game
you want to charge up your e passive and aa the enemy adc if you can, try not to take too much poke, you can use your flay to get them closer and activate your runes and then aa aswell. Just try to land hooks and flay them and do your thing. More than anything try not to die or let your adc die.

Mid Game
Items are very important, as long as your getting gold and not far behind if you land a good hook on an enemy carry you should q again to go towards the enemy and then flay them / their team and box them in with your ult. You can also throw your lantern to one of your tanks to bring them into the fray with you. Now you let your team slap them silly while you tank up the damage.

Late Game
Same with mid game, but try to always stay with your carries, try not to let anyone get picked off, use your lantern to save your teammates. Focus on objectives like dragons, baron, and towers.

Tip:
You can hook minions or jgl monsters and then q again to go towards them to escape enemy onslought
You can throw your lantern in the fog of war to give little vision in the area (good to throw in bushes to see if any big bad cat (Rengar) is hiding)
Your hook has a small delay, so throw where you think the enemy will walk, or flash.
DO NOT get tilted or discouraged b/c you can't land your hooks. Observe the enemy bot laners pathing and how they react when they see you throwing your hook and use it to anticipate there movements as the game progresses

Thresh is fun! Keep practicing and you might just be able to adequately suicide or get your adc killed, thats the best part of playing support. Killing your adc if he disrespects you :) Have fun summoners!