Hi Lentin,
All depends on the sound you want, and on your budget. Many people on YouTube just use the mic of their webcam or camcorder (or even cell phone), but that doesn't always sound too good.
You can also plug the guitar directly in your computer or digital recorder, but that will probably not be a very realistic sound either.
Of course you can also use a good microphone, which is maybe the best option.
I usually plug my guitar (which has an Fishman Matrix pickup) via the Fishman Aura into a small Boss digital recorder. At the same time, I record the image with my cheap camera (which is intended to make photographs, but has a basic video function). The internal mic of this camera is very bad, that is why I record the sound separately.
Then I put sound and image together with Windows Movie Maker, and try to synchronize as best as I can.
I don't do any EQ or whatever, so it's still not a perfect sound, but it is quickly done and good enough for YouTube.
Here is an example :
http://www.youtube.com/watch?v=gd-8mWfYgFE
Some have posted amazing videos here that look and sound much better than this one, so I hope they will tell us their "secrets"
