Go easy at first, your wrists and elbows are going to be sore. Watch for hyperextension if you get the bag swinging. You will have a new appreciation for those 20 minute fight scenes in the movies.
If you are going to kick also, don't try to go for the head. Keep your kicks mid-section level at first and work your way up. Roundhouse kicks that land on the top of your foot will make the front of your ankles pretty tender too in the beginning. For the most part your body will tell you how much it will take. Just be sure to listen.
You will get a real workout. BTW, get a good jump rope and skip some to warm up. Remember to stretch.