Best way of toning muscle without losing too much weight?

I have a body mass index of 17.6 which is underweight. I'm 20 years old and I'm 5'3" and weigh 99lbs. I don't think I'm too skinny though, I don't have bones that poke out. I think my problem is lack of muscle, I don't exercise that much and I'm quite weak. I want to tone my muscles, especially around the buttock's area, I think it's too saggy, even though it's not that big. What exercises should I do? I've been doing some occasional swimming, bike riding and brisk walks. Any other suggestions?
Best way of toning muscle without losing too much weight?
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