You will tan even if you wear a sunblock. I wear some when ever I go outside and I still tan. If you are quite pale and have not seen a lot of sun then get something slightly stronger so you have more protection. The SPF is the amount of UV radiation required to cause sunburn on skin with sunblock on, relative to the amount required without sunblock. So, wearing a sunscreen with SPF 50, your skin wont burn until it has been exposed to 50 times the amount of solar energy that would normally cause it to burn. The amount of solar energy you are exposed to depends not only on the amount of time you spend in the sun, but also the time of day.