Dermatologists will recommend using nothing less than SPF30, but it depends on how long you spend in the sun and the type of skin you have.
If you burn easily, I'd recommend starting with SPF30+, laying out for a couple hours, and when you get a bit of color to you, drop down the SPF.
A good way to tell if you're starting to burn is to press down on your skin with a finger -- the spot you press will turn whiter briefly, and it will take longer to do so if you're burnt, and should probably make you think about going inside.
Whatever you do, do NOT use anything less than SPF15, tanning oil, or try to get your tanning done in 1 day.