If someone starts acting hostile towards you, ALWAYS throw the first punch and ask questions later. A lot of people think it's legally smarter to wait for the other person to swing first, but in most jurisdictions just the threat of violence is considered assault and that's grounds to defend yourself. Opt for a quick punch in the nose as your opener vs. a huge haymaker.

Keep your hands up at all times to protect yourself if it turns into an actual fight.

If you are outnumbered, you should be running and not fighting. Same goes for if your opponent has any sort of weapon.

If you are in a fight and someone grabs you around the waist, make yourself dead weight immediately because unless they are a wrestler or know BJJ (in which case you are megafucked regardless) they will try to slam you. Even better, don't give them a chance to grab you around the waist.

Overall, everyone should get some kind of fight training if they can afford it. Gyms will be filled with chuds but you don't have to be friends with them and you can leave at any time. Even a couple of months of BJJ or Muay Thai will give you a huge advantage if you ever find yourself in a real situation.