Whats the best way to slim down arms and legs?

I've been working out now for a week, but so far I've just used the tredmill, stationary bicycle, and done sit ups and push ups along with lifting 2lb weights doing repetitions (3 sets of 10) but I want to know whats the best way to slim down arms and legs without building too much muscle, I'm scared that the fat I have in my arms or legs will harden up like and turn into big yuckie muscle! What is good for just slimming them down?