You'll probably have to get separate microphones for the guitar and the voice; there's not really a good way to have one microphone cover both. You could also comsider getting a pickup installed in the guitar, which will be useful down the road (for gigs, etc.) Of course, there's always the classic multi-track solution: record the guitar, then record the voice over it. Not as spontaneous, but easier with just one mike.

I have a similar problem. I've been goofing around in GarageBand on my computer, and all I have is a headset with microphone. When I try to record the harp, I have to bend my head down to the soundboard to really get the tone, which is extremely ergonomically unfriendly. ;) I do plan to get a harp pickup soon...just haven't got 'round to it yet.

Good luck!

Cheers,

Molly