Depends on the color of your skin and how long you expect to be out in the sun.
I use a sunscreen with SPF (sun protection factor) 30 because sometimes I burn pretty easily. Most common ones are 15, 3, and 50 but I think you might be able to get even stronger ones for mega sensitive people.
Oh the other thing is you shouldn't be out in the sun for more than 2 hours without putting more sunscreen on.