Ideally, everything on your iPod should already be in your iTunes library. Then, you should back up your iTunes library, in case your computer's drive fails.
You cannot use iTunes to transfer song files from iPod to computer, except for songs purchased from the iTunes Store. However, there are third-party methods and utilities that can transfer from iPod to computer. If you do an Internet search on something like "ipod transfer," you should get some links. (The CNET article seems to be popular).