Don't forget that you need to add a lot of extra protein in to your diet.
If you are relatively thin then you'll need to increase your daily calorie intake with the correct percentage of Protein, Carb and Fat. There are some complex calculations to work out the correct ratioon this. A good sports nutrition book will put you on the right track. If you wish to build muscle mas, then without the correct diet you just aren't going to achieve it without the correct diet.

A good website for pointers (although he's mainly trying to sell you his book) is

http://www.musclegaintips.com/