How to increase general strength and tone in females?

I'm a fourteen year old girl, and I've had some...Unhealthier methods of "weight loss" in the past, although I DEFINITELY don't need to lose weight. I'm actually a little underweight, (5"10 and 126lbs) and I'm very worried about gaining weight through fat, so I've decided to try and gain some in muscle weight. How should I go about this? I'm not sure what workouts I should do, and when, in order to increase my strength. All I really know is that I'll have to work out at least 3 times each week, but aside from that I'm not really sure what to do...I'm eating healthily, too. n_n