Well first of all, try to find its owner. If it was your iPod that got lost, wouldn't you want it back?
If that doesn't work though, you should know the following:
1) For some iPods, you need to delete all of the songs on them in order to add your own songs or edit existing songs (like deleting them individually).
2) The iPod might not be compatible if it was formatted for a different type of computer. (For example, if it was used with a Mac in the past, it probably won't work with a PC [running Windows] unless if you delete everything on it. However, I remember reading somewhere that PC-formatted iPods can work with Macs without deleting stuff, but not the other way around.)
3) You'll need a music playing program called iTunes to add/remove/edit songs and other stuff on the iPod. If you don't have it already, it's available as a free download here (don't worry, it doesn't have virus or stuff like that): http://www.apple.com/itunes/download/
4) Here's a link to the page on Apple's website with the official iPod manuals (also free): http://support.apple.com/manuals/#ipod
5) You need a cable to connect the iPod to your computer if you want to do this. Check out Apple's website (apple.com) or go to an Apple store and ask where you can get a cable.
6) Important! Once you get iTunes to open your connected iPod, if it says something like "Sync this iPod with this computer instead of another?", it will delete everything on the iPod if you say yes!!! Saying no won't let you add/delete/edit the songs and possibly even other stuff on the iPod. You won't be able to get songs from the iPod to the computer that aren't already there this way without special programs.