Some rules to stick by:
1) Increase your instake of protein, a lot.
2) Try and stick to about the same amount of carbs as you instake protein, if you do a lot of exercise.
3) Reduce the amount of fat you take in as much as possible, but you still need some. Aim to have about 8-10% body fat.
You need to do (1) because that's what builds muscle. You need to do (2) because if you don't have enough carbs and you're using lots of energy in your routines, your body will start using the protein (by converting it into stuff it can use as carbs). And (3) because you'll only see 6-pack abs etc. if you have a low fat count.
Make sure you eat plenty of fruit and veg too though; the vitamins etc. will really help your body in this process.
Most people can only take in (and use) about 30g of protein in a single intake, before it starts getting converted to carbs and then fat (unless you're doing an insane amount of exercise). This is 1 tin of tuna (which is great BTW, because it's almost pure protein). But don't eat too much of one thing. Tuna for example contains small amount of mercury. Too many tins a week could result in health problems.
Oh, and what kind of training are you doing? If you haven't started isometrics, start now. Bullworkers are very good at helping with isometrics.