whats the best way to change valve springs,how do you do it with the heads on ?

Use an air compressor to force air into the cylinders through the spark plug hole to keep the valves closed and then use a valve spring remover to get the retainers out and swap the springs.

Yep. Get a compression tester. That will allow you to connect a air hose.

You might also want to place a magnet next to each spring you are removing so you don't lose the keepers. The keepers hold the spring on. Once you compress the spring, you will need to pull them out before you take the spring out. There are 2 per spring, and together they make a circle.

Right. I've found that grabbing one of those telescoping magnet tools used from retrieving parts you drop into the engine bay works very well for this purpose.

Out of curiosity, why are you replacing your valve springs?

i want to put er rockers in and was told that you should put new springs in as well

Yes, it is a good idea. LS6 springs or #105s