First is water resistance. You don't want the sunscreen coming off when you sweat or go in the water.
Second, look for an sun protection factor, or SPF, of at least 15. The SPF rating is calculated by comparing the amount of time needed to produce sunburn on sunscreen-protected skin to the amount of time needed to cause sunburn on unprotected skin.
Finally, you need broad-spectrum protection. Look at the label. You want to see the following ingredients: benzophenones (oxybenzone), cinnamates (octylmethyl cinnamate and cinoxate), sulisobenzone, salicylates, titanium oxide, zinc oxide and avobenzone. If sunscreen doesn't have these ingredients, it probably only filters UVB light, which is the major culprit for sunburn and skin cancer, but not UVA rays, which are responsible for premature aging and the development of skin cancer. Needless to say, you want a sunscreen with those UVA-blocking ingredients.