Here is the advice I usually offer to complete beginners who are living pretty inactive lives now.
It gets you through about three months of improving your overall strength, fitness and flexibility. After that you can make good use of your own dumbells, barbells, or join a gym. But do that first and then come back to us
Once you've done that general physical preparation, then for strength you need about an hour three times a week, for fitness and flexibility together about the same. Usually closer to 30 minutes, though - depends how you do things. An hour a day, tops, with one day off a week. If you watch any tv or play any computer games or anything like that, then you have that time spare.
If you can get into the habit of regular exercise at 15, and keep at it, you will have a very fit and strong future ahead of you. If you become like Arnold Schwarzenegger in anything less than ten years of very heavy and dedicated training in combination with drugs, you can come back and abuse us. Bodybuilding is a slow process - though beginners, especially young males, can put quite a bit of weight on quickly.