As a skinny guy, what body building exercises can I do at home without the need of expensive equipment?

So as a skinny guy, besides the eating more deal, what are some building exercises that I can do to increase my muscle mass and tone?

I cant afford the expensive dumbbells or training equipment, but I dont mind getting creative with stuff lying around my house (like a backpack or a pillowcase to use as weights.)

Go army style. Try sit-ups and push-ups. You can hold your backback out infront of you with your arm at a 90 degree angle. Then, curl it up towards your head. Try experimenting with different amount of books etc.