Remember, your goal is to escape not to win. However, you must not be afraid to hurt an attacker. All kicks and blows must be forcefull and should be aimed at vulnerable areas such as the groin, eyes, or instep. Special self-defence skills such as judo or karate are popular. If you are proficient in such techniques, they can be very effective, but proficiency requires practice.
Some people carry weapons such as guns, knives, or spray chemicals to ward off attackers.
Unless you are trained and not affraid to use these weapons, they can be extremely dangerous. The attacker might be able to turn them against you. If you believe you might get hurt defending yourself or if you're afraid to fight back, don't. Sexual assault is still an assault and still a crime, even if you don't have a single cut or bruise. If attacked, escape.