Good resource for Ubuntu, includes how to lock and unlock the root user. As well as I believe some software administration tips.
And to use apt you dont need to seperately download the software, apt will take take of that for you. What you may need to download seperate due to copyright issues is the libdvdcss library, which is the DVD decryption library.
Anyways the basic rundown is...
>apt-cache search <Name of Package here>
should return a list of all packages matching the string entered above for name of package, so typing xine will return a list of all packages that are in the apt cache of the repositories you have set up. Lots of big words there, basicly you just need to know the command for right now, Ill get more into other things later.
>apt-get install <Name of Package>
This will actually download and install a prebuilt binary for the package, but you HAVE to use the name of the package as found using the search command. It should take care of everything else for you short of installing symbolic links(aka shortcuts) anywhere you might want them(Desktop if you use it for example, or menu, though I think Ubuntu sets up apt to install in the gnome menu as well, I dont know about kubuntu)
Another option though that I believe comes preinstalled in ubuntu is to use synaptic. Synaptic is a GUI for apt and can be a little easier to understand, and as I said I think it is included as part of the default installation.
Have fun, post up more questions you might get.