This is really nice, Jane.
Even if you don't have any programs for sound, there is a lot you can do to give yourself more control over the mixing.
Try recording the guitar by itself to start. Once you have a good take that you are happy with, you can use the tools in Windows Sound Recorder to trim any silence so that the guitar starts right up on cue.
Once you have this part to your satisfaction, get your headphones on and turn off your speakers. Run your guitar part and sing along, recording from your un-muted mic. Since you are recording on the mic and you have headphones, you'll get a recording of only your voice. If you have trouble working with the headphones at first, just slip one side off your ears so you can hear yourself.
Again, trim the wav file to your satisfaction. Make a backup of both clips, and start playing with the normalize feature. This will permit you to change the level of the clips, to bring down the guitar and bring up the voice.
Once you think you have it, use "mix paste" to blend the two together. The matching shouldn't be too hard if you've trimmed both clips.
This will help you control your levels a LOT better
If you want to take the time to do it, just make the two wavs and mail me. You can send them to me and I'll clean them up, add a few effects, and mix them down for you with my audio tools. It will be fun and you won't believe how good you sound.
EDIT: Oops, a couple of things if you want to try this. #1: converting to an mp3 is fine. #2 In this case, don't trim the clip, make sure there is at least three seconds (and five is better) of silence before the music begins. The reason for this is because then I can get a profile of the noise that is inherent in your computer and make it go away.