Ok, you want to lose weight while not building any more muscle. I say any more, because once you start losing weight, the muscles will start to show themselves more.
I don't know why people don't know this already but, there are two steps to losing weight.
1) Diet....and
2) ...Exercise
To lose weight you will have to change your diet so that you are burning more calories than you take in per day. Stop drinking soda if you do. Don't eat sugary foods. Cutting out sugar alone should make a big difference. Oh yea, no more alcohol either.
You want to be doing cardiovascular exercises because they will help you burn more calories, become healthier, and shave off weight without building a lot of muscle. Walking, Running, Swimming, Jumping Jacks, Skip rope, etc. Always stretch your muscles before a work-out.
On a side note, some girls have muscles. It's not a bad thing to have.