How to get a slow horse to lope/gallop?

Cues to get your horse to change gaits and make transitions should be done with your body position, seat and legs. As you progress with your riding, your horse should begin to respond more appropriately. If you do not know how to properly communicate with your horse, it is not reasonable to expect the proper response. Make sense? If you are new to riding, it is probably not a good idea to move into a gallop, most especially since you say you can't stop your horse. This does not sound safe. Take your time, be patient. You have just begun this process. You stop your horse with cues from your body, not by pulling on their mouth with the reins. If having difficulty stopping, you need to try to use a one rein stop. Get someone to help you. For most, riding is a commitment for life and no matter how long you ride, you never stop learning. You didn't mention age or what level of training your horse has so it is difficult to make too many comments. Again, be safe, be patient and enjoy yourself. You can't accomplish everything in one day. Good luck.