Some really great responses here, especially the ones that emphasise not fighting. Apart from the very good reasons they gave (jail being just one) the most important reason has not yet been covered: none of the technical advice here is going to matter for you when something goes down. Fighting is like any other skill in life - unless you do it a lot, you will suck at it. Having no training at all, I can tell you right now the first time something goes down you will have a whole host of physiological responses that will prevent you from thinking clearly and logically. This is before the fight even starts. Your heart rate will go through the roof and odds are you will get tunnel vision and auditory exclusion.
As soon as your body kicks in to fight or flight mode, you won't be thinking "don't go to the ground" or "poke his eye" etc. You won't be thinking about what combination to throw and you sure as hell won't turn into Jason Bourne. You may even just freeze completely. Unless you have done real scenario training or a decent amount of sport fighting, you won't know what your response will be.
My advice? Avoid what I call the triple S:
If you really want fighting skills, take up MMA and go 3 times a week. It will give you a big bag of tools that will work in most situations. Fighting is a perishable skill, it's something you have to practice a lot and even then, it is at best an insurance policy should something happen. How good that insurance policy is corresponds to how hard/often you train. But it is never a guarantee. Also, do a few self defence seminars run by reputable people that will teach use of force and de escalation.
Let me preface this in saying that I am 40 years old and it has been years since I have been in a proper fight and I am glad of that fact. I have read all the posting on this topic and am quite surprised by many of the responses. The whole "avoid violence at all costs, violence is unmanly, etc,etc" replies baffle me. I feel that I had a gentlemanly upbringing and the need to properly address violence prepared me for my life. Violence for violence sake (after boyhood, which I will discuss later) was frowned upon but violence was accepted and even expected in the following cases:
1. Threat to family
2. Threat to property
3. Threat to Honor or reputation
As a boy or young adult, violence to establish pecking order and to establish physical dominance was acceptable if it wasnt done in a bullying way. Also the violence of youth prepared one for later in life when it may be necessary for protection. I was taught to box by my father and taught to brawl by my older brothers and cousins and later friends. By the time it was necessary for me to defend myself against a real threat I was prepared. Besides the physical lessons, I was taught alot of the emotional traits needed to succeed in most altercations. Mainly, remain calm. Secondly, it doesn't hurt as bad if you remain focused on your goal. And lastly, fight to win. Period. If you are going to worry about prison or hospital (for either party) then you will lose the fight. You must be ready to do what it takes to win. If it is hand to hand then you have to use any advantadge you have. If it goes to weapons you have to accept that and do what it takes to win. Cold detachment with complete focus on completeing the task at hand.
The reason there's been so many 'avoid violence' posts is because nowadays unlike when you were in your late teens/early 20's people are FAR quicker to involve the police.
I won't start fights, I'll defend myself but even then that can be a 'legal grey area'.
Example... someone throws a punch at me I retaliate (defending myself) and kick this guys ass badly... Now I didn't instigate the fight in this senario, I was merely defending myself but say this guy, his buddies, someone in a crowd decides to call the police. Depending on how things play out (witness testimonies etc.) even though I was defending myself in theory an argument could be made that because I kicked this guys ass, and did more than just say shove him away or whatever. then I could be charged with assault and battery.
It's stupid (IMO) but would I be willing to risk jail time just to teach a drunk a lesson? Probably not.
Also keep in mind my example is assuming "drunken asshole throws a punch for no real reason just cuz he's a drunk asshole" if you're in serious physical danger/fear for your life then yes.. throw caution to the wind do what it takes to make sure HE goes down and not you.