The first thing that you want to look at when buying sunscreen is the SPF. This is the number that lets you know how long you should be able to safely stay in the sun before the product begins to not work. Most sunscreens have an SPF of about 50. This means that you can usually stay in the sun for about an hour before reapplying the product. An SPF of 30 to 50 is usually sufficient. Make sure the sunscreen offers UVA and UVB protection.
There are two kinds of sunscreen that you will see on store shelves. One is a lotion and is usually the most common. Lotions can better be applied to the skin as you can easily see where you're rubbing the product, getting it into the skin in even amounts. Spray-on sunscreens are good for those who don't plan on staying outside for long periods of time or those who are looking to stay outside for tanning. Keep in mind that spraying a sunscreen sometimes means that you aren't able to get the product evenly on the skin as the droplets are scattered.
Look at the label to see if the sunscreen is for daily uses or for those who are more active. Sunscreens for daily uses usually only provide protection for a short time. Products for those who are active are usually water resistant and last for longer periods since you'll be outside longer. You also want to find a sunscreen that has a higher protection if it's being used on children. Their skin is fragile and burns easily, so children need a higher SPF than adults.