The full version has more tips.
When battling opponents, KEEP MOVING. If you have the rocket launcher you can afford to stand still for a few seconds, but in MOST cases you must keep moving
Get lots of health and armor.
If you are in the water and see someone floating on the surface, blow 'em away. You can see them but they can't see you unless they come down.
Beware of starting points so you are not telefragged.
Console commands can be entered on the command line preceded with a "+" (e.g. quake +map <mapname>).
Use "bind" to bind/map keys to commands.  For example, "bind f12 screenshot" will take a screenshot each time you press f12.
At the console prompt, type "sv_gravity 200".  This will change the gravitational constant of all objects.  It will be like you're on the moon.
In Windows 3.1 or Windows 95, you can use Alt+Tab to switch applications.
Live a happy life and do a random act of kindness today.