For unreal tournament 2003, mount disc 3, and "cd" to your cdrom drives directory in a terminal, and then type "su -m", then enter your root password. type "ls". There should be something like "ut2003_install.sh" or something like that. Type "./(installname.sh)", and it will launch the install program. When its done installing, type "ut2003" into a console to launch it. Now the disc labels are a little screwed up during the installation, I believe "Play Disc" is number 1, and "Disc 2" is really disk 3, or something like that. Just try the different cds when it asks you to switch until one works. :P
Im not sure about RTCW, but if it has a linux installer, its probobly installed the same way.
Half life is a little complicated to set up, so maybe you should get some more experience before you do that.
For help installing normal programs, theres 1000 threads on it. Just search "compiling from source" or "installing programs" or something like that.