I don't know how other guys do it, but I generally like to do a bunch of different kinds of exercises. So what I do, is do so many "REPS" (repititions, ie-# push ups.) then so many sit ups and so many other exercises, then I go back and start over and do the second set. On the second set, I start out and do the exact same number of push ups, sit up, and the rest. You want to adjust it so that you don't do 50 push ups the first set and then can't do 10 on the second. You want to make them evened out. The number of sets is how many times you go through the whole group of exercises.