How can I be slim fit?

Stacyzee
I am already slim 5'6 120 lbs. My skin is tight that people usually think I work out when I do not. Although I am small I have a round butt that sticks out.
Although, I look ideal I've been wanting to take better care of my health.
What exercises will better tone me and not make me lose my butt?
I want a stronger look.
So far I do 20 min on the eliptical. / I alternate between that and the step up work out I do for 20 min.
90 squats.
30 arm lifts.
30 arm curls.
I hope this doesn't make me lose weigh because that's not what I want to do.
I'm new to working out and have always been blessed with a fast metabolism.
How can I be slim fit?
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