If you’re doing much more than about fifteen reps, you probably aren’t building muscle like you’re aiming for. Try lifting more weight for fewer reps. This will both build and tone your muscles. Aim for anywhere between 8–12 reps for about 3–4 sets per exercise. Concentrate on your form on each repetition. If you are lifting heavy weights improperly, you won’t receive the desired effects, and you’ll risk injury.
There is little need to worry about becoming “bulky” like the female bodybuilders you sometimes see. It takes lots and lots of work to build a body like that (and, more often than not, lots of steroids). Women typically don’t have testosterone in high enough concentrations to build bulky muscles.
Focus on large muscle movements: chest, back, legs. The benefit of these exercises is that they are compound movements. For example, pushups—as Shilolo mentioned—work your pectoral muscles, but also your triceps, shoulders, and core muscles. Chin-ups, work your upper back and bicep muscles. Squats and lunges work pretty much every muscle in your legs and rear end.
I’ve found the best results in my workouts doing bodyweight-bearing exercises such as these. Vary each exercise. Do pushups on your knees if you have to, but varying your hand position focuses on different muscles. Narrow grip or diamond pushups focus on your triceps, while a wider grip focuses more on your pectorals. There are many different types of pullups you can do, and altering your grip on those also changes which muscles in your back and arms are getting more work.
Vary your workouts every four to six weeks, to avoid “plateauing.” There are lots of ideas online for finding different exercises. Remember that your diet is a big part of building muscle. I’ve heard that you should eat about as much protein, in grams, as you weigh in pounds. Of course, that means that you’ll have to find a bathroom scale somewhere, but if you weigh 100 lbs, you should aim for about 100 grams of (lean) protein per day to build muscle.
There is some truth to the old saying, “No pain, no gain.” In order to build muscle, you must break it down. You should do each set to near failure. In other words, on each set, perform reps until you can no longer do another rep with good form. It should burn, but not be painful. When you’re first starting out, you’ll be really sore the next day, and, if you did a really good workout, you won’t want to get out of bed two days later. But it’ll be a good type of soreness (I like it anyway). But give yourself enough time to rest between workouts. Your muscles grow when you are resting—not when you are working them. It works well if you do one muscle group every other day. Monday: chest. Wednesday: back. Friday: legs. Good luck!