Have a flat tire on your bicycle? No problem... let High Altitude 2U help you out with their video tutorial.
The best way to fix a flat is to avoid them, But how do you avoid flats?
Keep your tires properly inflated. A bicycle tire should be pumped up to 40 - 60 psi. Put it toward the high side when you're on the road, and ease off the pressure when you're off road. If your tires are too low you're likely to have pinch flats from the sides of the rim. They look like a snake has bitten your tube. If your tires are over inflated they can explode.
Sometimes those sharp objects that puncture tires are too small to be seen as you ride and you'll ride over them, causing flats. There are products which will help prevent flats. "Slime" is non-toxic goop that you put inside the tube before you inflate. The slime automatically seals small punctures. Slime only works with standard (Schraeder) tubes, because you can't remove the valve stems on Presta tubes. Tire Liners are another product that help prevent flat tires. A tire liner is a long strip of tough material that you put between the tube and the tire. There are also thorn-resistant tubes, they are thicker than your average tubes to provide protection against flat tires.
If you think you have a flat, stop riding. Even if there's still some air pressure in the tire, stop riding your bike, otherwise you'll damage the wheel rims.
Empty the remaining air from the tube by removing the valve cap by pressing on the valve.
Remove the wheel from the bike by releasing the brakes and the quick release at the hub. If your wheels are not quick release but rather held on by nuts, loosen then with a 15mm wrench. Try to avoid using a crescent wrench because if it's not set correctly, a crescent wrench can strip the nut.