Choosing the right sun protection factor (SPF) is really a matter of listening to doctors' recommendations regarding skincare. Most dermatologists recommend an SPF of at least 15-30, although there are sunscreens with an SPF of up to 100. There is little evidence that these products protect much better than lower SPF items.
Your unique skin type plays a major role in how badly and how quickly you will burn in the sun. The SPF on most sunscreens also gives an indication of how much longer you can stay outdoors without burning. For instance, if you would normally begin to develop a sunburn after being outside for 20 minutes, a sunscreen with a sun protection factor of 30 means that you could stay outside 30 times longer without damage. This system can be deceptive, however, since higher SPF numbers do not seem to offer much more protection than lower ones. For example, products with a sun protection factor of more than 30-45 will not protect you much longer than items with lower protection.
Aside from choosing a sunscreen with high enough sun protection, you should also choose products that you don't mind using. This means choosing lotions or sprays that feel good against the skin and which have a fragrance you enjoy. If you don't like wearing a product, then you are much less likely to practice proper sun safety.
The most common types of sunscreen are lotions and sprays. Both of these offer similar protection, so choose the one that is most convenient for you to apply. Sprays are often easier for young children since they have trouble holding still long enough for lotion to be applied and rubbed in sufficiently. Just make sure you use a sunscreen that is designed for kids or for sensitive skin types, and that you avoid getting it in their eyes. You can also choose sunscreens without scents if you have problems with fragrances.
It's also important that you apply sunscreen every two hours, no matter what SPF level you choose. The chemicals in lotions and sprays both lose effectiveness over time, so reapplication helps to ensure maximum protection if you will be outside for longer periods. If you will be swimming or otherwise engaging in activities involving water, make sure to choose a water-resistant or waterproof sunscreen. Water resistant products do wash off, however, and should be reapplied as often as every hour.