join a gym/buy some decent home equipment
Find a training partner who will motivate you (if they already train and have some knowledge thats a bonus)
Eat
Train hard
Get enough sleep
When starting out you will have mewbie gains, which basically means whatever you do you will gain size at a fast rate for the first few months. But after that you a long/hard road to gaining muscle mass.
Diet should be protein 40%, carbs 40%, Fat 20%