Well if the iPod doesn't show up in iTunes at all, then you won't be able to reformat it unfortunately.
If it does show up, click on the line in the left-hand panel that has the name of your iPod, and it'll bring up kind of a main menu for it which shows the capacity, software version, serial number, etc. There will be a button named "Restore" in the window - click that.
A box will come up asking you if you're sure you want to restore to default settings, you will lose all the stuff on your iPod, etc. - click "Restore". It'll do the reformatting, and then give you a little notice saying it's done. If it works, the iPod will reappear in the left-hand panel (it may take a while), and then it will ask you to name your iPod and choose the sync settings. After that, you should be able to use it just like before.