Number of Exercises
First of all we should look at the number of exercises that you are going to do per muscle group. There are nine muscle groups that we can distinguish just for simplicity; calves, quadriceps, hamstrings, abs, chest, back, shoulders, biceps and triceps. The best way to work a muscle is to work all of the muscle fibers and the only way to do this is to do exercises that work the muscle from different angles so that you can tax each part of the muscle. Often advanced body builders will do three or four exercises for each muscle group. This means for example that for your shoulders you would do front lateral raises for the front deltoid, side lateral raises for the medial deltoid and reverse flyes for the rear deltoid.
Number of sets per exercise
Next we will look at the number of sets you will do for each muscle group. I believe that it is important to not overtrain your muscles and the best way to do this is to reduce the number of sets that you do in each workout. The way that I get the sets going when I have had a long layoff is to do one set for each exercise and three exercises for each muscle group. You will be able to gauge when it is time to increase the number of sets by listening to your body. Your muscles quickly adapt to any kind of weight training and so after a couple of weeks you will be able to add a set to one of the exercises and then another set to another exercise maybe a week later and as you get stronger you will feel you have more in you after you have rested a bit after a set and can come back with another set to further push your muscles.
Number of reps per set
Lastly is the number of reps you should do in each set. The prevailing wisdom ever since I can remember is that for a big muscle group like you quads, chest, back, biceps, triceps or hamstrings you should do four to eight reps per set after you have done a warm-up set. For the smaller muscle groups like calves or shoulders you should do a higher number of reps per set this means you should do eight to twelve reps per set. The way to do these sets initially anyway is to warm-up and then do a light weight set for a high number of reps and then move the weight up for each set after and drop the number of reps and for advanced weight lifters the last set is a lower weight for a higher number of reps until you can not move the muscle anymore.