welcome to AA first off.
to answer you question about elimination, a player is eliminated by getting hit with a BB but it is your responsibility to call your hit and remove your self from the game or walk back to a re spawn point. Airsoft stops being fun when people aren't calling their shots.
Now how a player goes about eliminating another player depends entirely on the scenario. Most games require you to call your hit if your hit any where on the body, other scenarios may call a body / head shot an elimination. its all up to the planners of the scenario your at.
As a safety note: most of the games being held in Arizona along with the rest of the nation require your weapon system to produce no more then one joule of energy (for AEGs), which at the chrono is roughly 400 feet per second with a bb that weighs 0.20 grams.
about your choice in AEG, id recommend echo 1. i play an echo 1 SCAR-L and i love it!
Again, welcome to AA
-jeremy