If you want to learn how to defend yourself, the ideal is to go to a self-defence class. In the meantime here are some tips from the Cosmopolitan website: Rape Escape Tips  

IF THINGS GO WRONG

• Act immediately: Trust your intuition and get away if possible. Don't give in to a person's sexual demands in the hope that you can divert him/her later on.
• Don't feel obliged to stay somewhere if you feel awkward.
• If you're really scared, excuse yourself, say you're going to the bathroom and leave through a back entrance.
• 'Never worry or feel embarrassed about your behaviour,' say the experts. Rather be rude or lie, than risk your safety. We are taught, especially as women to be respectful, gentle and accommodating - when it comes to your safety, forget the rules, trust your instincts. 'Your safety is much more important than one person's opinion of you.'
• Stay calm. Try to think clearly about all your options. Your brain is your best weapon.
• Passive resistance. You may be able to discourage the attacker by talking. Persuade him/her not to commit the assault by making him/her see you as a friend.
• Active resistance. If you are not afraid to hurt someone, hit and kick hard-this gives you the opportunity to escape. However, fighting back may anger the attacker and cause him/her to attack more brutally. Self-defense training can make you more confident and improve your physical strength. Training is effective, but it takes continuous practice. It is not a substitute for common sense and awareness.
• Submitting. Do whatever you have to do to keep yourself safe. If you feel your life is in danger, your best option may be to submit. Submitting does not mean you consented. The assault is not your fault.
• REMEMBER:  If you leave move to a public place with lots of people if at all possible.

General Strategies for Safety

• Avoid secluded places.
• Tell someone where you are going and have a way to get home.
• Know your limits and observe them. Alcohol and drugs impair everyone's perception and judgement.
• When going out with someone new, don't feel you have to go alone. Go on a group date or meet in a public place.
• Just because you know someone online, does not mean that you can trust them. It is very easy for them to lie online - do not meet them alone.
• Be aware of date rape drugs. Don't accept beverages from open containers and don't leave your drink unattended. Watch your drink carefully; do not leave your drink unattended or accept one from someone you do not know well or trust.
• If you left your drink unattended, rather take it with to the bathroom and throw it out - don't drink it.
• Trust your instincts. If you feel something is wrong, do not ignore your feelings.
• Be cautious inviting someone into your home or going to someone else's home. Three out of 5 sexual assaults occur in the victim's home or the home of an acquaintance.
• Assert yourself. If you do not like something that your partner is doing, tell them clearly and firmly to stop.
• Use a buddy system when you go to parties. Before you go to the party, agree with your friends when you are going to leave, or under what circumstances you will leave each other.
• Agree upon signals to give your friends that will indicate needing an "escape" or a way out if a situation becomes uncomfortable. Don't be afraid to intervene if you think your friend is in a bad situation.  
• Avoid individuals who:
  • don't listen to you
  • ignore your personal space boundaries
  • make you feel guilty or accuse you of being "uptight" for resisting sexual advances
  • express sexists attitudes and jokes
  • act jealous or possessive