That will depend on a number of factors, including country of citizenship and the amount and origin of any funding. There are visas specific to certain countries and these are always the ones to look at first because they are often the easiest to obtain. Other potential visas include the E-1, E-2, and O-1. And sometimes it just makes sense to remain employed by the founder's current H-1B employer and file a concurrent part-time H-1B through the startup, which at least allows the founder to work openly and with valid work authorization for his or her company.