you should include the obvious 'gl' 'gg' 'glahf'
common abbreviations for each game type " stb, lmoh/lmoth, bop, ctf, terries, etc
common map abbreviations... gimble,venice,mazz,clash,chest, etc
common net terms: ping, lag, drop, etc
common unit name abbreviations: zerk,jman,duff/dorf,arc,myrk,etc
and don't forget: 'check in'
if you want more words from which to choose the most common/useful, you could always check out the myth dictionary:
http://mything.org/index.php?a=words-all-letter
ps. This is probably obvious to you, but I would recommend making sure you have your focus for your newbie guide well defined. Otherwise if you aren't careful and keep adding topics and expanding sections, you may end up with something like mything.org. e.g. TONS of information, but too much for the newbie to use or find efficiently. From experience with FAQs and maintaining the Myth FAQ for a couple of years, I can say that the larger a guide gets the more unwieldly and unused it will become. It can be a hard balance to find - the balance between brevity-ease-of-use and having all the info you think should be in the guide - but if you keep the balance in mind it will probably help you keep your guide as useful as possible.