In the simplest, Thresh is one of the most well sought after and impacting supports throughout League. His kit performs in both highly offensive and defensive compositions, giving him the fluctuations between a support and support carry role. The learning curve is mild, but high level plays will need a complete understanding and experience in the role itself.

The pressure that Thresh can lean in lane can be terribly acute, not only does his hook and flay cause movement issues, but his lantern can be used to bring in, or just bluff,a jungle gank. Be sure to save up the flay passive in order to use the relic shield's active. Along with good lantern positions and relic shield hits, an ADC can lane longer vs a non-sustain bot. The main point I have learned with years of experience with and against Thresh, is never be too liberal in using the lantern. Always save it if you are pushed up, for it can save a life more than once. And be sure to use it when being pushed against, a few auto-attacks shielded under tower can change the tides of fights.
Always maintain pressure with the advantage that Thresh possesses, under tower or pushing an enemy tower, the hook, flay, and lantern can solely control a lane.

Picks: Along with major CC supports, Thresh can cause major damage by hooking enemy carries prior to objectives. Warding is key to this point, but simple observation can help sway a fight from 5 v 5 to 5 v 4.

Skirmishes: Thresh has major peel and engage potential, be sure to use both to the advantage in small skirmishes. Protect your carries with flays and lanterns while trying to hook major threats for a possible kill.

Teamfights: Thresh's output in teamfights resembles very much that of the skirmish, higher risk and control must be taken though, in order to be successful in a brawled out fight. A good ultimate can split and catch out champions, slowing and giving time for your team to collapse. The major part of this role is knowing when to go in on a good hook and flay (this comes with time as many things do, so be patient).

Thresh can change the tides of war and bring up picks to help a team win a game. Be sure to be mindful of your actions and always protect your carries over yourself.

"He will win who knows when to fight and when not to fight." - Sun Tzu