Here is a nice illustrated guide: http://hubpages.com/hub/How-to-Changa-a-Bike-Tube---an-Illustrated-Guide

Christmas time is when kids get new bikes. Maybe your teen will be getting a new mountain bike?

Make sure you have the right kind of pump for that bike. Many high-end mountain bicycles or road bicycles will have a presta valve tube -- so you can just use your standard tube/pump.

Another alternative is to get a brass presta valve adapter.

I really like my Presta tubes, ever since my first road bike years ago. However, I would recommend against using adaters ... they always cause trouble. Instead, the best bike pump you can afford, and you should be ready to go.

A good pump will let you pump past 100 lbs.