How do you get to valor lake?

In the Valor Lakefront.

it is northeast of pastoria city and west of sunyshore city

You do pretty much nothing except battle team Galactic. You do learn though that team galactic is trying to capture the Pokemon of all three lakes. This is when you travel to lake acuity.

by pastoria city

You need to have beaten Byron and gone to the meeting in the Canalave City library. If you mean catching Azelf, you can surf there after you beat the Elite Four.