Attack Damage Carry Thresh consists of AD, AS, Crit chance, And Tons of damage.

I suggest you take 9 xGreater Mark of Desolation
9x Greater Seal of Strength
9x Greater Glyph of Strength
3x Greater Quintessence of Desolation.

For ranked play, I advise that you practice a fair bit beforehand, But when you thin you're ready, You can try it in ranked, Just make sure you get a ward when you play!

Pros: Good lane bully
can do lots of damage
can CC
Can have sustain
can adc really well.
Cons: Not as tanky as usual.

When jungling, I say take smite, Start at red, Move to wraiths, Move to wolves, move to blue, Clear golems, gank when needed!

Secure kills, I know it's a bit stupid sounding, but now that you're the adc, you'll need kills! get fed and you'll be unstoppable!

Make sure to get the max amount of CS without dying. the more, the better, Collect souls when possible.