This depends in part on how your computer is set up.
Do you all log in with the same account (or even not log in at all) or do you all have your own accounts?
If you have your own accounts and you can't already see the music that one member of your family has loaded into iTunes then it should "just work".
If you all share the computer then things will still work fine.
However what I would recomend is that you set each iPod to be manually managed.
As a "default" (well I think it's the default) an iPod will make sure that the music on your iPod and in iTunes is kept in sync. So if you buy a new song then everyone will get it, and if you delete something then everyone will have it deleted.
If you manually manage each iPod then that doesn't happen. You copy the music you want onto each iPod and they can be as different as you like.
Each DS can have whatever music they want that way and you can have your own selection on your iPhone.
The instructions for manually managing the content are here:
support.apple.com/kb/HT1535
Each iPod should also have it's own settings for managing things like contacts, diaries and photos. So just set them to be what you want each one to be.