Hey, its a 1st person shooter but its AMAZING,
its completely free- you have to download something but it is safe, virus free, and makes the game work.
Try:
quakelive.com
It has about 40 maps, 10 guns, and you can pratice against computer or online- for me i know play against friends all the time, in a clan.
Give it some time to get used to it (say a week) cos to begin with you dont really know all the tricks or maps, and play like a noob

but once you know how to rocket jump or use the guns, and know the maps, it is sooooo addictive
Have fun

Cathy