Choose a sunscreen with a Sun Protection Factor (SPF) of at least 15, and reapply it at least every 2-3 hours. Do not assume that a sunscreen with an SPF higher than 30 will keep you extra safe; most additional protection is marginal. Keep in mind that even the strongest sunscreen cannot completely protect you from sun damage. If you start to feel symptoms such as redness, pain, blistering, tingling or itching on your skin, dizziness, nausea or headache, you may be experiencing sun poisoning, which is a severe form of sunburn. Get out of the sun immediately, cool your skin with a wet cloth, and drink plenty of water.
When you do buy sunscreen, look for one that protects against both UVA and UVB rays. If you are going to participate in sports or go swimming, choose a waterproof sunscreen, but remember that you will still need to reapply it when you are finished with your activity.
Also remember, clouds do not block UV rays, so make sure you use sunscreen even if it is cloudy outside.