When you start training, you are going to find that your Gi and belt will become loose. Some looseness is good as it will help you gain range of motion, but at some point it can become more of a hassle as you end up wrestling with your Gi and belt as well as your opponent. You will also want to avoid stopping to readjust everything and taking away from your training time. Here is an excellent tutorial on how to tie your belt in a manner that will keep it in place.
Also…TIE YOUR PANTS!