If you're naturally skinny and are going balls out to put on mass, i would look at diet before supplements.

Basically eat everything and anything you can lay your hands on! 3000-4000 calories a day or more if you can.

Supplement wise, there are loads of weight gainers on the market. BSN True Mass is one that gets good reviews. I prefer a simple whey protein powder though but i only need to sniff a mars bar and i put on weight.

go to www.myprotein.com/uk and have a look, probably the best priced supplements. They do loads of weight gainers etc...

Put in promo code MP147724 for 5% or 10% off your first order too