Note: The following is based on my experience running a youtube channel 5 or so years ago, thus content might not be completely valid/up-to-date.
You've got a couple options here, all depending on how professional you'd like your setup to be.
the first is quicktime. Open the app and press command+control+N, you should be greeted with a new screen recording dialog to configure audio devices blablabla. Click on start, then when your done the stop button in menu_extras>quicktime. By default there's no way of recording your voice and system audio simultaneously, but nothing loopback can't accomplish.
I personally went with an app called screenflow. It's pretty expensive though, $130. If your able to scrape up the cash or crack or whatever, I can't think of anything better. We're talking extremely configurable with keystrokes, you've even got a handy audio-pluggin which records not only microphone but optionally system audio. After recording, you'll be taken to a basic but quite accessible editor. As I said above, it's been a while since I've touched video recording, when in doubt go with the first option since I can confirm it works. Is anyone else able to give insight to screenflow these days?