A tip in order to survive in matches is not to find instantly an enemy to kill. Some players will wait for you to pursue them to kill you, and while they aren't "chasing", they collect all the items they can, unless (or not) if they find you in their way.
First than nothing, go searching for better weapons than MG, and lots of health and armor. And then, you can use RL and GL to cause splash damage, (and hit directly) LG and MG for constant hitting, RG to hit directly, SG in close combat, PG and BFG to spam (

) etc.
Also, item timing (also called Map Control, I guess) is a good thing.
And another thing, if you got Q3A, download some demos (some of them may require some mod -or version of it- like CPMA or OSP) from sites like ClanBase or QuakeCon, and watch them in order to learn some things.