Thresh is a pretty straight forward champion. Great CC, Shield, and Tank. His combos are not that hard, only ones you need are the E+W, mentioned at the Ezreal counter (you flay first, then you hook since he's slowed and you have more chances of landing the hook) and the Q and E straight after (this makes his hook somewhat faster, I tried it and it's pretty OP).

If you can roam as any Support, you'll win almost every game. Thresh has a very big roam capability. He can easily Stun and Slow the opponent, making for an easy kill. You'll practically become a second support, which is pretty neat for somebody who might need a lot of help and his jungler can't come. Also, his tankyness makes for better diving.
To sum up, roam all you can to win the game. If your adc can't win or he's not that skilled, tell him to def and roam (hopefully you'll have a better chance winning mid than bot).

Predicting is pretty hard. My advice is that you learn the player's pathing. If you miss hooks, it will only make you learn more about how he moves when he gets scared. That way you can land the perfect hooks that BunnyFufuu makes in his montages.
The Other thing that you should remember is the player's flash cooldown. As soon as he pops it, you can calculate his cooldown (4 min 45 sec) with the time in your game. That way, you can always know when to predict the flash and when not to. It's something small that gives you a lot of advantages.

I hope that you liked my guide. I'm a small 250k Thresh main and I've been maining him for quite a while. The information I got was due to experimentation and a lot of time spent on watching montages and guides of other pros.