Nothing says summertime like going out in the sun and getting horribly burned, right? Wrong!
You should be wearing sunscreen, and chances are, you've been using it incorrectly this whole time.
As long as your sunscreen is broad spectrum, water resistant, and SPF30+, you should be fine. You can ignore all the other fancy words like "antioxidant" and "natural" on the bottle. Just make sure that you like the feel and smell of the lotion. Sunscreen is only potent for three years, so if you've had the bottle longer, get a new one just to be safe.
A recommended amount is 1.5 ounces (about a full shot glass) which is a lot, but this is for your entire body—including under clothes—and don't miss common areas like eyelids, brows, feet, and backs of ears. If you have a good coating all over your skin, and still have some left in the shot glass, you're probably fine.
What about spray sunscreen? For that, you want to spray a layer on your skin, make sure it is visibly shiny and wet, and then rub it in completely. Just spraying a thin mist on your body offers very little protection.
You should wear sunscreen whenever you go outside. Even if you'll be under an umbrella. Even if you'll be mostly in the car. Even if it's cloudy out. Even if you have a darker complexion. Everyone should be wearing sunscreen whenever they're outside!
You'll also want to apply the lotion 15 minutes before going outside, so it's fully absorbed once you're exposed to the sun. If you put it on once you're outside, you are opening yourself up to danger for those first few minutes. And don't forget to reapply every two hours, especially if you are sweating, swimming, or toweling off. Now go enjoy your summer!