Installation Problems

I am trying to upgrade from 10.3 to 10.4, and I'm pretty new to Mac so I'm not 100% what is going on, but when I try to boot off the DVD I can hear it being read but then I just come back to the normal logon as if it didn't boot from the DVD. I have also tried the optional installation and it reads all the packages and then a prompt appears saying it needs to check a program before it can install. I then get an error message saying Mail cannot be installed.

Can someone please help me cause I am very lost...

I don't want to undermine your intelligence, but you did say that you were new to Macs, so:
I had a few problems with my Install Disk not booting up when I clicked the "Restart and Install" option on the disk when it mounts. So, to boot from a CD/DVD, put the disk in and make sure it mounts on your desktop, then restart. As soon as you see the screen go black, start holding the "C" key on your keyboard. It should boot from DVD. Try this and see if it works. If you were already doing this, then nevermind.