I am a 17 year old female about 5'6" to 5'7" and weigh about 120/125 lbs, and am skinny so I don't want to lose any weight. I am active in sports, but I am not where I want to be and would like to be more successful. So how do I get get toned without losing weight? Where do I start?
The term you're looking for is "strength training." Success in sports, when excluding skill and strategy in the sport itself, is heavily dependent on your strength and power.
Lifting weights is the primary method of strength training for sport. Ask your coaches to help you with a lifting program. If they don't know how or say it's not important, then find one yourself. Make sure to avoid programs that focus on conditioning or bodybuilding, because those are not applicable to your situation.