Getting moving before your workout, no matter what your level will improve your results. Physically a warm up will get your circulation going, which will allow you to work at a higher level. Psychologically, a warm up will shift your focus to your workout, and off such things as your workday or the drive to gym. Here are the simple things you should do, and should not do as part of a 5-10 minute warm up for fitness.
Do perform light cardiovascular activity such as riding a bike, elliptical or brisk treadmill walking. Perform this at an intensity that allows you to feel the muscles getting warm, and where you might have a little difficulty carrying on a conversation. Five minutes for most people is plenty!
Do not perform static stretching as your strength can go down up to 9% in the hour following the stretching. Static stretching gets the body to relax, not get charged up and can actually decrease muscular temperature and circulation.
Do add some dynamic movements like jumping jacks, arm swings, torso twists, hops, skips and the like as they prepare your muscles for quick movements, increase flexibility and heighten your nervous system for more powerful movements. This is a major factor in reducing injuries.
Do not perform the first few exercises at full intensity. Even with the warm up, it will take another 5-10 minutes for your body to reach peak readiness, so ease into your workout. If performing lifting, then work on some major movements like bench press, leg press, etc. If performing athletic activities try to combine coordination and movement, like catching passes in football. Ease in, and your body will thank you!