The first rule of self-defense is DON'T BE THERE! If there is a situation that you are not comfortable being in, leave. Always be aware of your surroundings. Know who is around you at all times and assess the threat they may pose. Not everyone is going to harm you, but anyone could try.
There is a universal rule that every parent tells their children: don't talk to strangers! In the real world, as adults, we have to interact with people we do not know. Self-defense is so much more than fighting. Let your instincts protect you. If a situation seems unnerving or a person is behaving strangely, remove yourself from that situation. Never be afraid to walk away from someone or something that makes you unfortable.