This will get more oxygen in your bloodstream and to your muscles. It also prevent -or at least reduce- muscle soreness after your workout.
A typical warm up would be to do is 5 push ups and 5 sit ups. After you do both, rest for 30 seconds. Then 10 of each, rest 30 seconds again. 20 of each, rest. Then work back down to the 10 and 5. After that, stretch your torso and do squats and stretch those muscles again
Source // WikiHow