You need the ESTA then, it takes about 2mins to complete. Make sure you use the official FREE site as there are some cowboy sites out there that charge. It is valid for 2 years, you do not have to fill in the hotel name or flight nunber bit as obviously that will be different each time you enter the country over the next 2 years.
https://esta.cbp.dhs.gov/
A hint: take a screen grab of the last page as it doesn't acually email you any confimrations
Hint 2: If you are cruising then I hope you are using this site
http://boards.cruisecritic.com/ and have joined the Roll Call for your cruise to meet your fellow cruisers before you board.