What are some healthy things you can do in order to gain weight?

If you want to gain weight (in muscle) make sure you focus your workouts on weight-bearing exercises. So go for an aerobic warmup, like the treadmill or stairmaster, then hit the weights room. The first exercises you do should be the ones that involve the whole body, either squats, deadlifts or bench press. These are the ones that produce hormones that cause muscle growth throughout the whole body. Follow these with the smaller moves that isolate muscles like bicep curls, crunches, rows, calf raises, triceps kickbacks, etc. After this have a source of protein straight away, like a protein shake or if you can't afford it then use skim milk and egg white (it works just as well). A couple of hours later you should be pretty hungry so have a good nutritious meal with plenty of protein. Make sure you get plenty of sleep after all this too. Make sure you have a food routine, eating at least 3 meals a day and not missing any. Anything that is missing - quality food, exercise, or sleep will prevent you from gaining muscle mass. Your whole lifestyle is just as important as the exercises you choose.