Hi,
I've been playing support for 2 seasons now and I chose Thresh over any other, I hope that I explain how you can play him or atleast some build and runes to help you do it.
I'm not a high elo player or anything, still, I think I know a thing or two about this champion.
I'm sorry if something is terrebly wrong, feel free to comment.
I hope you enjoy it :)

I use mostly this runes, more for the fact that they have a good amount of resistences and has a little AD for the poke.

My alternative to the offensive Thresh rune page is the one bellow. I like this one because of the movement speed, it's pretty good vs a team with high mobility. And the only difference for the other page is the 0 extra AD and 1 less point for Armor.

For spells I take it situational.
<-- Has to be allways there.

<-- I pick it more vs an enemy with high damage or with great mobility (Ex.: Shyvana, Kalista).

<-- I take ignite more when the enemy either has a lot of heals (like Soraka) or when they don't have dodging spells (Ex.: you don't need exhaust vs a MF for the only escape she has is flash and if she uses it even if she runs away it's a good worth)