There is a number of ways to approch this, however, I belive that Nemethor sugestion on starting with modding is probably the best idea to get started.
My best advice on where to go looking and hang around will be
www.3dbuzz.com . 3dbuzz contains an amazing amount of tutorials for modding games, however there is a lot of VTM's that are quite big in filesize, so an alternative is that you buy Unreal Tournament 2004: Editors choice (Its quite cheap) which contain all the unreal VTM's so all you have to do is to sit down and watch.
Then of course there is the game art section on
www.cgtalk.com