it depends on ur skin tone and how easily you burn. If you burn easy id start out with atleast 15 and flip over at around five minutes. the darker you get the longer you can go. If you dont burn almost ever i use just straight baby oil its the fastest way to tan
To suntan, locate an area where you will lay out. It should be a fine place where the sun shines. Apply the sunscreen all times. Using the sun protection factor of about 15 or even higher is suggested. Get more tips at http://www.ehow.com/how_2058228_get-suntan.html.