Don't rely on your hook exclusively. It is a big part of Thresh's kit, but not all of it!
A good Thresh is not defined only by his "Madlife" hooks. If you're not that great at line skill-shots, you can still be a good Thresh (with practice). When you see a hook opportunity you're not entirely confident in, flash flay and then hook to secure it!

One of the most important parts of the game as Thresh is your laning. Go for harass, but only when you are able to. Pay attention to the abilities your enemy support uses and take note of them. As long as you win trades, your lane should be an easy one. Thresh is one of the lane bullies in the support role, but careful for laners such as Blitzcrank who can hook you under turret. Your primary objective in this lane is to keep your adc alive or to get your adc fed. Depending on the carry, you may be able to feed the hell out of them if it is a heavily farm-reliant champion play a bit more passive than usual.

- Leona: This is skill matchup. Save flay for her xenith, hook her when you can and try to force her to burn xenith. Leona's weakness is she can't go out once she goes in, so rely on that.
- Lulu: Disengage, polymorph, range + high damage. Lulu is annoying. Period. Just farm if you're not confident in getting kills in the lane.
- Janna: She has a lot of disengage, but this is also a skill matchup. If you can hook her, you'll probably kill her.
- Morgana: This is also a skill matchup. Get inside her head and force her to spellshield the wrong target. Easy hooks and easy flays = easy kills.
- Zyra: She does so much f***ing damage. Just hook her and she'll die though.

Tbh a lot of Thresh's build relies on personal preference. I prefer a tankier variant of Thresh that rushes cooldown reduction. My final build is example #1. Another friend of mine prefers something like version 2 which is a less tanky, but more utility focused Thresh.

My tip is to rush the items that build into final items rather than the final item. (i.e Sightstone -> Lucidity Boots -> Eye of the Equinox -> Glacial Shroud -> Kindlegem -> Whatever you want).

Don't just focus on killing the enemy team. Peel for your adc as much as you can, even if you die it's okay as long as your adc survives. Go for hooks and flays on the enemy carries. Also don't be afraid to try to make a play. Flash hook into box into flay is one of the deadliest support combos in the game!

WARDS
LOVE THEM, BUY THEM, USE THEM.
Vision wins games. Some good ward spots are near dragon and deep wards in their jungle.

Roaming mid to help out that laner can always help your team as long as they are informed as to what you are doing and your adc doesn't go up to far. Careful with roaming, and be sure to communicate. Pay attention to the summoner spells in other lanes as well as your own to know where you can roam.