Do cardio anything, running, swimming, cycling, aerobics, etc. You need to lose weight over your whole body to lose fat in your thighs and butt.
If your weight is already fine, then you can try a body sculpting class, running, lower body weight machines, Pilates focussing on the lower body, etc.
You won't lose the fat so much with the strengthening excercizes, but the muscles under the fat would get firm and toned.
I think this is the wrong section for ths post, but I'm not a moderator. I think this is where people put fitness articles.