Alright, I got you bro.

First things first, you're going to want to the first punch. If you're lucky the guy has no idea what he's doing and he'll get "up in your face" or try pushing you, this is great because as soon as he's up close you should be getting the first hit in as square on his jaw as you can.

AIM FOR THE JAW.

Everyone saying to take cheap shots is correct, but in a fist fight between two unskilled people you won't have much opportunity unless you guys go to the floor, in which case bite the fuck out of him and hook his body everywhere you can. If he starts choking you don't try to pull his hands away, drive a hand up in between his hands (with both hands if you can like the superman pose) and jerk like you're breaststroking. If you can't visualise that google how to get out of being choked.

You didn't say how long you have, but if you have even a week, thats a hell of a long time to learn a bit of the basics. Forget about fitness because it's too late for that, but at least learn how to throw a proper hook with both hands and the jab n cross on a bag. Sounds pointless but even without training it can help.

Stay away from kicks, you'll just fuck it up.

Keep in mind he's going to bring his friends or a weapon, you should do the same. Keep a knife in your back pocket but don't think of using it unless he has one himself, in which case either back down and run or gank him in the throat like the faggot fuck he is.

Now, depending on time, let's talk drugs assuming you're in the US. Just your basic adderall (amphetamine sulphate) can do wonders for your physical and mental resilience in a fight, cocaine is excellent due it's numbing effects of the face and all around effects, but the gold standerd is cheque drops or halo if you have access to those steroids somehow (/r/darknetmarkets if you have time). Any of these will give you the edge you need especially if this is your first fight and you're lacking balls, but the last two are the gold standard of ferociousness.

Now you need to think of the legal side of things. If you're in an abandoned parking lot or park you don't need to worry, but if you're in a mcdonalds parking lot you need to think about the legal consequenses of what's about to go down, you might break this dudes jaw for all you know. If there's cameras something I recommend is holding your hands up or forward as if you're surrendering or telling him to back off, while simultaniouslly egging him on. That way on the security camera it looks like you're telling him to back off, but in reality you're telling him you're going cripple him and fuck his mother.