Weight training and Swimming (laps etc) are both excellent for buidling muscle. YMCA's are pretty affordable. If you like a sport ,why not join a team.
Google ,protein and carbohydrate foods and eat plenty,pasta,rice,potatoes ,bananas,breads etc. Drink whole milk.
Although fatty foods add wieght they dont have the nutrition you need for building good healthy cells (the ones ya want to bulk up)
Avoid stress: Mid 20's is still young in terms of biological development,in a couple of years, hormones kick up a gear and you will find the body naturally will fill out and by eating and getting some excercise you will be toned as well.