Gaining weight is simple:
I started out at 6'4" 155, and three months later i was 6'4" 185 and gaining. Thirty pounds of mostly muscle ain't half bad
You don't want to gain fat; you want to gain muscle.
Diet: as much high-protein, high-iron food as you can get. MEAT: steak, chicken, whatever. Milk, peanut butter, cottage cheese; they're all good Eggs, rice, beans good too. Really just make time to eat, no matter what it is.
Workout: the best workouts for gaining bulk are multi-joint exercizes, and you want to do as much weight as you can for no more than 3 sets of 8 (say first set of seven or eight, second of five or six, third of three or four) reps. Give yourself at least a minute between sets. Don't do too little weight--if you can do more, do it. Oh, get somebody to work out with you for spotting.
The best lifts:
Squats. Squats are the most important. Just make sure you work up slowly so you don't screw up your joints, and make sure you get down far enough.
Dead lift: put a bar on the floor. If you can, try to make sure you're a couple inches above the floor (say standing on a plate). with legs straight (not locked) and back straight (think as though you're sticking your chest out) bend at the hips, grab the bar, and stand up. Whatever you do, don't jerk, just pull.
Weighted situps: put a plate of however much weight you can handle on your chest, and sit up
dips. Dips get your tricep, your shoulder, and some of your chest. Good stuff.
Bench, shoulder press, upright rows, pullups all good.
Try to add a little weight each time you work out, and don't do more than 10 repetitions a set.
If you work out and you eat, you'll gain weight.