Some people want to gain weight!

So, you're one of the lucky people who has room to gain weight, eh? Yeah I can help you with that.

I'm going to assume that if you want to gain weight, then you want to gain muscle. If you didn't, then you'd know simply to head to McDonald's and have at it. Most important part: diet.

This is delicate because it depends on your age, your metabolism, and your target. I always find 2,500 calories to be a safe starting number. If you don't have a scale, then buy one so you can make sure this is not too much/enough. More specific:

-1 gram of protein per desired pound of lean body weight.
-1/2 gram of fat per desired pound of lean body weight.
-300-400 grams of complex carbs.

Now, if this is not enough calories and you want quicker gains, then up the protein and the complex carbs just a little bit at a time and see what happens. If this is too many calories, then drop the carbs before anything else. It's up to you to keep track and find the proper measurement: there is no universal number here!

No you need to exercise! First thing will be weightlifting for muscle mass. Basic rule of thumb will be to aim for 8-12 reps for each set you do. Feel free, however, to throw in some sets where you aim for 6-7 reps as this will help with gains in strength, allowing higher weights to be used in your 8-12 rep sets. So I would start out, for the first 2 months, aiming for 8-12 reps. Once you start to learn more about weightlifting and your abilities, then throw in 1-2 days each week where you aim for 6-7 reps.

Also - stick to the basics that I laid out before: 48 hours rest per muscle group, drink lots of water, and eat that protein. You should be lifting at least 3 days per week, but preferably 4.

Your cardio should be minimal, but you should still have some in there. Keep it to 3 days of cardio (falling on days where you aren't lifting, preferably), and keep it to 30 minute sessions. If you're taking in that much protein, fat, and calories, then your heart will need some exercise to keep up with the extra weight and subsequent cholesterol. Just go for a brisk walk on an incline tredmill, or a light eliptical session, or a light cycling session. Does not have to be intense, and it does not have to be long.

Good luck.