Do you mean "I want to record a video of me playing but I want the sound to be a little better?"
Can your camera do real-time into your computer with USB?
Are you singing and playing or just playing?
Electric guitar or Acoustic?
The simplest and cheapest way will be something like the aforementioned Focusrite Scarlett 2i2 and the mic you have. If you're singing, you might consider one more mic like a Shure SM58 for $125 or so. Use your 'fairly nice mic (please tell us what it is)' and the other mic. One goes into each side of the Focusrite (or whatever thing you choose that has at least 2 inputs.)
Download OBS freeware, which will allow you to combine the realtime video from your camera and the noises you make with the mic. You can adjust the tone of the mics 'a little bit', likely enough to make you happy without pulling out your hair.
You can record the completed video right in OBS. You can also stream with it if you want to.
that's the easy answer for maybe a few hundred bucks.
If your camera can't do real time to the computer you can buy a nice webcam or a GoPro that will do it.
