Like doing it. Some men gain muscle easily doing resistance training, others gain more slowly; most women prefer lean and sinewy to "big". Anabolic steroid use is cheating, but I recommend creatine supplementation to get bigger. The key is to find some way to make yourself enjoy lifting weights or doing other resistance exercise. Have fun.
Eat and exercise. You need to focus your exercise regimen more on weight training than aerobics. Work out with heavier weights and fewer reps per set - around four reps per set. Keep good form with your exercises to minimize chance of injury. As well, make sure you eats lots of food, and focus on complex carbohydrates and lean protein sources.