Wondering what I should do at the gym?

I'm a 25 year old female, relatively healthy just looking to lose about 15 lbs, I'm currently 135 and 5'4". I really just want to tone up, get back to being a little bit healthier. I just recently started back at college so I get to use the campus rec center for free which has any kind of machine you could think of, basketball courts, tennis courts, indoor pool, etc. So far I've just been really busting it cardio wise, using the stationary bike and elliptical. Alternating doing crunches/sit ups every other day as well but I feel like maybe I could do more if I just knew what to do. I was thinking about swimming laps every morning before work Tuesday's and Thursday's but really I'm just kind of doing stuff and not really knowing what it's helping.

I'd like to know what I can do to tone my stomach up, to drop about 15 lbs and gain some more muscle.