Start of with the assumption you need a visa.
For many people the visa waiver scheme applies - you have to be the right nationality, have no criminal record, no serious illness, a passport that is either machine readable or has a chip in it (depending on the date it was issued), be visiting for less than 90 days and have a ticket home or at least out of North America and be travelling on an approved airline/cruise ship.
You can read all about it here
http://www.travel.state.gov/visa/temp/without/ without_1990.html#2
and get horribly confused in the process.