Here's a primer on fighting. Hopefully it will flash through your brain 4 seconds before you are in a confrontation that you can't get out of. Hopefully it will help.
Who will win a fight consists of several factors: who trained the most, who is heavier, who is faster and who wants it the most (intent)
Training: This is going to give you better aim, speed and muscle memory for any move that you may try. Muscle memory comes from the synaptic pathways learning a specific set of instructions, it also increases speed since it is enhanced by a lower (more animal) part of the brain. Train now and you may be able to walk away from a fight even easier. This you CAN do beforehand.
Weight: Force is defined as Mass times Speed. The heavier the mass is the less speed that one needs to build to create the damage needed. This means generally a heavier person can punch harder. Weightier people are usually slower too. Size up your opponent.
Speed: You can't increase your mass at the onset of a fight, but you can definitely increase your speed. Here are some ways. Be soft the whole time. Stay loose and your muscles will move faster both for offensive reasons and defensive reasons. Only make a fist right before you hit your opponent and only lock your arm when you make contact. Loosen those muscles up by doing a quick crouch right before. It will even reset your spine. This is of course if you have the time.
Heart: The person with the gravest intent will be the one most likely to win. If you go into a fight just wanting to get back out of it you will most likely wake up in a hospital. Even if you don't want to be in a fight in the first place you have to go into it with the intent of ripping someones head off. You might want to practice this before a fight too.
Some more things: Don't move until you have to, and don't stop moving until it is all over. Choose soft targets; throats, eyes, stomach. Always block, don't "trade shots." Don't just be on the defensive, be the aggressor once it has started. It is harder to get hit if you are the one hitting. There is no real morality or rules to a fight. Cheat, cheat the whole fucking time. Anything is a weapon. Getting punched is not really painful, more of a shock really. Use your elbows and knees... a lot!
Edit: My instructor reminds me that the first one to hit is also usually the winner. Getting hit demoralizes people and their intent goes out the window. Getting hit also can cause a damage that limits. That's why he's the instructor. He's a bad Mofo!