OK.
The easiest thing to do is to see if you can manage with playlists. Don't mess around creating another library or an account unless this doesn't do what you want it to. Creating a new account/log in would be the last thing I would do if your machine is all happily set up to be shared between the pair of you and this is the only "problem" you're having.
Set yourself up a playlist in iTunes. Click the little plus button down at the bottom left and call it "Cheese's Playlist" or something. Then copy what music you want into it.
Then plug in the iPod Shuffle.
Follow the instructions here:
support.apple.com/kb/ht1719
If you've got a 3rd of 4th generation (spot it by the photos) then follow the instructions labelled "Syncing music automatically to iPod shuffle (3rd and 4th generation)". Select to sync only the play list you've just made.
If you've got a 1st or 2nd generation then follow the bit labelled "Syncing music using Autofill to iPod shuffle (1st and 2nd generation)" and select your new playlist for the "Autofill from" bit.
That should do it, just the music from your playlist will get onto your Shuffle.
You can also do it manually, and again instructions for that are on the same page above, which means you can drag or drop whatever songs you want onto the Shuffle without needing a playlist.