Assuming that you already have itunes installed and your iPod isn't sync'd with your library:
1. Open up itunes
2. Plug the ipod usb cable into the computer's usb port and plug your ipod in at the other end.
If you don't know which is the usb port on your computer a symbol next to it should look like this:
3. Wait for your ipod to show up in itunes
4. Select the songs you want to put on your ipod and drag them over into the "Music" folder on your ipod and wait for it to load.
5. When you are done, make sure you eject your ipod
There's an eject sign next to where it should show "(Your name)'s iPod"
That should be it
Depending on what kind of iPod you have there MIGHT be a different way to do it but I know this way works with the iPod mini, nano, and video.
If your iPod is synced to your library all you really have to do is plug it in and it adds all of the songs in your itunes library to your ipod.http://support.apple.com/kb/ht1351
Hope I helped