Depends which distro you're using (and which package manager at that). For Debian-based distributions, the apt-get repositories list is in /etc/apt/sources.list. Simply add urls that you want (you can find some at www.apt-get.org).

If you're not using apt-get, well you'll just have to elaborate.

Well I keep getting the error when I apt-get telling me that there are no such files or directories on several of the urls I have, and it tells me to apt-get update to fix these problems. It tells me to apt-get update to get rid of these problems even when I apt-get update.

OK, what you'll want to do is add a few repositories from apt-get.org, however the site seems to be down at the moment.

In the meantime, check out the packages at http://www.debian.org/distrib/packages and download any .debs if you want to install a particular package. To install a .deb file, simply use the following syntax on the command line:

Hope this helps

I think it was only down because I was accessing the extremely long list of repositories, and they said that it should only be done in extreme circumstances, because it's that big.

Alright, I think this all helps... but what about the synaptic pkg manager? Doesn't that update my repositories itself? OR could it?

Synaptic Package Manager is basically a fancy GUI for apt-get. It does everything that apt-get can do, but you might want to try it and see if it automatically updates everything. And yes it can update repostiories.

One big question for solving this: What distribution are you using? And, what version?

If you've gone through and added repositories for other needs and failed to remove them, then they'll still be in the list. To get the list optimized, you should either remove these entries or comment them out (with a # sign)

First, open the list:
sudo gedit /etc/apt/sources.list

You'll want to remove anything you aren't currently using. The important ones are the distributions specific entries, such as the dapper (or edgy) universe and multiverse entries. Also, be sure that you don't have duplicates.

I'd recommend commenting different entries out rather than removing them, as it makes correcting mistakes, should you make them, much easier.

google for "sources.list ubuntu" (or whatever dist you have) theres loads of sample ones on the net you can cut and paste

Well, I am using Debian etch. The problem is that several of my entries apparently don't have proper lists on those sites for apt-get. I can't tell which entries these are that are giving me a shit-load of error messages every time. I liked the idea to search for samples, maybe I could go to the debian web-site and get a file from there to replace my sources.list.
Thanks for the help, everyone. I'm sure these ideas will help me out. I have a lot of things on my mind with this computer, so one down is nice.

i have debian etch i can give you a sources.list (set up for UK based mirrors with all debian repositories and security updates). Hint, dont use synaptic to modify sources.list, edit it by hand and it will get less mashed up

here is my sources.list


after copying this sources.list to /etc/apt you must do apt-get update and check for errors