Welcome to DRN!
1. Go to the what bike should I get subforum
2.Tell them about yourself ..weight, height, riding ability, type of riding ect...
3. Ask what to look out for if you don't know.
4. Decide if you (or your father too?) have any brand preferences.
5. Look around and buy your bike.
6. Buy at least a helmet, goggles, gloves and some type of boots before your first ride. Add knee and shin guards as soon as possible and always wear as much protective gear as you can afford.