Last, but definitely not the least, your nutrition. You are what you eat! Get enough protein. Get enough carbs, fats and other nutrients to support your body. The question is what is enough. If you are really serious about working out and is training hard then you need to consume about 1-2 gms of protein per pound of your body weight. Again, this is applicable only if you are training hard and lifting heavy. If you are looking to bulk up, then you need to eat a calorie surplus; meaning more carbs and fats. If you are trying to cut down, then eat at a calorie deficit. Try to make sure that your diet contains as much raw food as you can think of. Besides that, cook as much as you can and avoid processed/canned foods as much as possible. Have a cheat meal every week to keep yourself in the game and to not feel like you are punishing yourself.
It’s true that you want to gain some weight, but you want that weight to be muscle mass and not fat! Consume quality food like lean proteins, complex carbs and healthy fats and stay away from junk food, lots of sweets and fatty foods. The only time when you should consume fast absorbing carbohydrates (usually foods that have a lot of sugar or white flour) is right after your workout. Read more about what to eat to gain muscle in this article.
Figure out how to pay the bills. Bodybuilders don't make lots of money, so the idea of becoming a professional bodybuilder is a lot like the idea of become a professional poet or professional painter: it'll take a lot of your heart and soul, but you'll also need to figure out how to take care of the practical details. You've got to support your bodybuilding with some other kind of work to pay the bills.
One of the most misunderstood areas for new bodybuilders is nutrition. I talk to guys all the time that have no idea of their daily calorie intake, their daily protein intake, their carbohydrate intake. They have no idea of what types of foods they should be eating, or when they should be eating them. They don't know what supplements do what and what they should using. Let me refer readers to some of my other articles that detail these areas.