With Thresh you always have the advantage in a teamfight on 1. So, if you can convince your fellow team mates, go for an Invade.

You want to land a Death sentence(Q) on your target and wait your team to kill it. (A bit like blitz, with the difference that you not just pull that the target, but you also immobilize it.)

Thresh is a good support to any adc. Very tanky, very good at initiation and very much independent. As long as your team knows how to click on the lantern when you throw it on the ground, you will have no trouble saving them from almost impossible situations.

Your passive is your best friend. Collect as many souls as you can. (Tip: Your shielding ability, (Dark passage) the one throwing the lantern can also collect them. If you have enough mana, do not hesitate to throw it at places where it's too dangerous to walk in)

The explanation behind the runes is simple:
You get the movement speed for easier engaging and escaping.
You get the armor for more durability against their adc in bot lane.
You get the Scaling MR so late game not to worry in team fights.
And finally you get the Mana regen ones so you are safe to stay with your adc for long periods of time.

Pros

Strong initiator
Strong CC
Very tanky/Hard to kill
Good teamfight capabilities
Strong farmer, even as a support
Strong harrasing capabilities


Cons

Long cooldown on most abilities -> Easy to exploit if failing
High skill cap
Not very long range
Very weak versus champions that cannot be slowed or rooted

This is a very quick and hopefully clean guide for Thresh. It is also my first guide, so I will be happy to respond to comments and questions about it or the champion.

Thank you for reading, and let me know your thoughts, suggestions and comments :)

Best regards