It depends why you don't already have muscles. If you want to be ripped you need underlying muscle AND you need to shed some of that fat that hides those muscles. To shed fat, you need to burn up more calories than you take in, which means a) cardio/exercise b) eat lower calorie foods. A good way to do that without totally going nuts is to eat high fiber foods and cut out sugars, juice, soda, and white bread. For cardio/exercise, running or cycling (depending on your knees) for about 20-25 minutes at a time will work well.
Muscle-wise, you need to lift. That's about it. If your goal is fat-shedding, then you can ramp up the reps and use low-medium weight. If your pretty slim to begin with, then just go medium-high weight. Focusing on major muscle groups is a good way to kick-start your metabolism, which means you'll be burning more calories while rest, which means shedding excess fat. Select a weight that allows you to do 8-10 reps; if you're able to do 12 or more reps, increase the weight. For 4 sets and make the first set a bit lighter to allow your muscles to stretch.
SO, if you're slim, lift 2 days in a row, then take one day off for the next 2 weeks. If you've got some fat to shed, alternate running and lifting and take a day off every 4 days (if you can). Make sure to tweak your diet a bit. After you run, focus on proteins and go easy on carbs (fruits and veggies are fine). After you lift, eat a decent amount of sugar or white-bread (high-glycemic carbs) and a decent amount of protein (15-20 grams). The most important thing, I think, is to stay supportive of yourself and not to go so crazy that you begin to dislike your body. Looking at it like a game you're playing might be a good idea that prevents you from taking things too seriously.
