I am normally a size 1 or 3 in jeans. Does anyone know what size I would be in european jeans or jeans like true religion or Juicy?

A US 2 is a UK 4 - UK is approximately 2 sizes larger.

True Religion runs small and it's normally recommended that you buy one size larger.

I don't know about junior sizes.

Im from the UK.
Your size 0 is a size 4 in england.
It goes up in 2's.
So like US 0 = UK 4
In european sizes a size 36 is a 10 in UK sizes.
So a 36 would be a US 6.
And that also goes up in 2's.

I'm not so sure about that. I wear a size 2 to 4 american. When I'm closer to a size 4, I wear size 42 italian. French size for that I think is a 40. Then uk is different, too! If you can try it on, it's worth it.