How to gain weight and muscle (nutrition and diet)?

[newberry]

I'm 22, about 6' tall, and weigh around 125 pounds. Obviously very thin, but currently very active and eat well.
I'm planning on beginning some weight training, but I know diet is a huge factor in gaining muscle. Any ideas on what kind of nutrition intake I should be taking?

[GazzyG]

At under 9 stone and six feet tall, I'll tell you what people are going to answer with:

Eat EVERYTHING, haha!

[Off Road]

Eat EVERYTHING, haha!
I'll second that.

[tom183]

Eat EVERYTHING, haha!

I'll third it.