Hello!
I'm a Platinum 3 player and i wanted to share this guide with you guys. I will teach you the bacics of this champ. In my opinion Thresh is the best support right now. He's really fun to play and I hope u will enjoy him too.

Get theese to lower the damage from the AP poke.
More health. Who don't want that :D.
Same as greater marks of magic resist.
It gives you 12.78 armor so its good for Thresh

Goor
Take ignite if there is soraka in the enemy team for sure and if they don't have a tanky support. Ignite will lower the healby 50 % and it will secure the kills.

Take exhaust if they have a mobile assasin (Zed, Talon, Le blanc). It will save your adc if you use Exhaust on their assasin that jumps on your adc. It makes it easier to hook someone too.

Makes warding much easier.
Great item early game. Gives your adc a free shield that very often saves his life.
Buy it every game. AP aura shield and if u use it a shield like form the Face of the Mountain for everyone !!! YAY
But it every game too. It makes it even harder to get away from you.
Nice item. Reduces the enemy attack speed so its good and it gives you CDR so buy it almost every game.
Buy this item only if they have at least 2 mobile champions or EZREAL. Will speed you and your team up so u can catch your enemies.
Very good item if they have a lot of CC.
Good item too. It passive it's awansome. If u get 100 charges it gives you and your adc 20% AP and 50% crit chance for 8 seconds.

Laning phase
The enemies will mostly try to hide behind minions so the best idea is too hide in the bushes and try to grab them from there (lower reaction time).
But here's the most hard part. If u play Thresh you have to provide the enemy movement. At the begining it will be very hard but if u play more and more u will be getting very good at it.

Roaming
If your adc is going back to base or if you kill the enemy bot lane you should go roam to mid lane to help your mid laner kill the enemy mid laner. By the way you are going to get there you can place wards . But ONLY roam if they dont have a mobile mid laner. If u are going to try grab Le Blanc it will mostly end with a missed hook.

Mid-Late Game
In Mid and Late game your job is too catch their squishies (ADC or Mid Laner or Jungler if he's squishy). If u manage to do that your team will just end your job by killing him and saying "N1". Don't press you again if u catch their tanks couse it a waste. Always focus their main dmg first.