Setting up a voiceover studio at home is more complicated than you might think. For instance, in addition to a decent microphone, a way to get the sound from that decent microphone in to your computer, record that sound, edit that audio, and so forth and so on; you also need to be concerned with how your room sounds.
Not just are there sounds coming through from outside (though if there are, that is a problem that will have to be dealt with); but also what kind of coloration is your room adding to your recordings? And don’t imagine that slapping a bunch of acoustic foam up all over the place is the magic solution. Each room has its own unique acoustic characteristics and taming the challenges can be a time and money consuming process.
Recently I stumbled upon a wonderful web page with loads of information about Small Room Acoustics and Sound Theory. I hope you find some useful information there. I certainly have.
Update: Note, that page is geared toward music recording, but the same issues apply to recording voiceovers.