I agree with rum - Creatine will help increase your muscle mass.. it's probably one of the supplements that the most study has been done on. However, without correct diet and routine outside of taking creatine you probably aren't going to get too far.
When you do decide to add it to your regime, generally you'll start with 5g of Creatine monohydrate 5 times a day for 5 or 6 days. This is called "creatine loading". Once you are "loaded" you only need about 5g per day to maintain.
You can buy creatine monohydrate