Code3Arena is an excelent source for tutorials and on how the Q3a engine works;
http://code3arena.planetquake.gamespy.com/
I suggest you first view the articles, these will explain what entities are, what cgame/game/ui is, and lots of other stuff. http://code3arena.planetquake.gamespy.com/articles
Also, you might want to search this forum on existing topics of ‘starting a mod’.
And you might want to check out Fretn’s starters guide on how to make RTCW mods(simular to ET);
http://w.twwwo.be/rtcwmods/
If you still can’t figure out stuff, create a new topic ,preferably using a clear topic title, eg ‘[help] weapon declerations’, so other new coders can easily find the same thing you were once looking for.
And don’t forget to ask the right questions, if the question is formulated well, expierenced coders will be more interested in answering.
Good luck!
PS.
Just out of interest, what is your programming background/ what experience do you have?