if its electric , a small mixer or a di box will do the job ... Accoustic , well the only way i know is to buy a semi accoustic ... what soundcard are u using ??
I use a Seymour Duncan Woody pickup (about $80 at Musiciansfriend) for my Martin D-28. Soundcard is Terratec EWS88MT. I run it through the cheapest Behringer pre-amp mixer and use Vegas to record and Sound Forge to edit. Usually have to use heavy compression (SF) but can get some great acoustic sound.