For tax purposes it would be best to request to be taxed as an S-Corp. You can then incorporate as an LLC or a corporation.
Depending on if you have no money coming in, or a small amount of money, it may not be worth it to do payroll... but that's only for the smallest of businesses.
As others have mentioned, we've started with the C-Corp form because we think it will be what most of our users would prefer initially, but we'll look at expanding it to other forms.