To be able to defend yourself when attacked, read this how-to.

[edit] Steps

It's all about appearance. If you look tough while fighting, you're bound to do better. Wear clothes that look cool and intimidating at the same time.
When you talk, fake your voice, make yourself sound somewhat psychotic yet controlled and intelligent.
Don't take anything from people. If someone hits you and they're the same size HIT THEM BACK!!! DO NOT SHOW FEAR! If you back down, they're bound to come back later.
Never ever turn your back on a fight, always be on your toes.
In order to keep yourself from being hit, watch your opponent's shoulders whenever the shoulders move. Be ready to dodge, duck or weave, this will give you a quicker reaction time than watching the arm.
If you want to take your opponent down to the ground, first allow them to throw a punch, make sure you duck under, then quickly punch at the jugular notch.
- This is located right above the collar bone.