It is not abnormal for people, especially new students, to fall asleep while meditating. Some tips to help overcome this:
Do not meditate in bed and do not lay down.
Try mixing it up and do something different.

In addition, you are learning how to voluntarily change your brain waves during meditation just like when you fall asleep at night. This is a habit for most people and therefore sometimes we miss the sweet spot and go too deep into sleep. The best possible posture to use is to sit up in a chair or in the lotus position. If it keeps happening try sitting up without your back resting against anything. You might experience head bobs, which means you are changing your brain waves and that's a good sign. If it still persists, we recommend that you do the breath, pulling the mind out of the body, so that you have more energy in your brain.