There are a lot of sunscreens on the market, and not all do what they claim. Read the label, and pick a sunscreen with:

Broad spectrum sun protection (these protect against both UVA and UVB radiation)

SPF 30, or higher

Water resistant if you are swimming or boating.

SPF 30 blocks 97% of UV and SPF 50 98%. There really isn’t much difference between factor 30 and 50 in terms of protection. Its far more important to put enough on and reapply it regularly. How much should you apply? Often more than you think – an adult needs about a golf ball’s volume to cover their body.