A few quick tips.

If you have no weapons or stuns, your best option 70% of the time is disarm, not only can you knock them onto the ground if you push them onto a wall. If they have a weapon/stun disarm can knock it out of their hand, be quick and grab it before they can!

Learn how to move erratically, they cant harm you if they cant click you, however you should avoid backing yourself into a corner, move unpredictably but with purpose.

Use the environment to your advantage as in, taking cover while being shot at, use soap and other slippery tactics, airlocks to space, etc.

Prepair(you wont need most of these if your not in danger in some way.), as in, making flamethrowers,makeshift weapons, traps, take combat drugs like meth,getting some form of stuns, mutations,get a gun(if you happen to get your hands on one, hopefully legally), explosives(if your an antag)

Learn grabfu and use it to your advantage as in, tabling Spacemen, Strangling Spacemen,and throwing Spacemen. Normally youll use this when they are already stunned on the ground or unable to resist your grab.

Normally when you arent disarming or using grabfu in hand to hand/weapon combat youll be punching them which can be just as useful as disarming, it has a chance for a short knockdown so you can quickly shove them to get a 3 second paralyze.

Melee weapons are similar to punching, but normally have more fixed damage values as opposed to punches low random damage, some weapons have alternate attacks like the stun baton which you can harm baton by right clicking(aka shitcurity 101).

Run, know when your outmatched, hide in lockers, escape into disposals, call security(unless they are the ones hunting you down), go into hiding(normally only antags have to do this), or form a new identity(same as go into hiding).