I don't know how to physically defend myself?

Most martial arts teach the best way to fight is to avoid the fight. Nothing is more important than your safety. If you must fight though, take a martial art. Krav maga is perfect for the kind of situations you are in. it is a martial are made in isreal by their military. Also, brazilian jiu jitsu is good for grappeling. while most people can throw punches, the , minute you take them to the ground and get them into submissions they will leave you alone. :)