You incorporate before your first sale! Depending on your state you can incorporate yourself for a few hundred dollars (CA for example only requires a one page declaration and filing fee of $145, first years tax to the state of $800 is waived).

You do this for liability reasons.

Secondly equity should be split based on contribution. If you plan to take on capital partners you will all be diluted at that time.

This a complex question with many open ended issues. I advise speaking with someone in detail rather than making your decisions based on a few replies on this thread.

Best of luck
Damien