OK, these professional applications (GarageBand is really just a cut down version of a pro application) are incredibly confusing. The main problem is that the overall user model is hard to understand, so the whole thing seems super mysterious. It would be so much useful if they just gave an explanation of how the internals work, then it would not be so mysterious, so here are the quick lessons on how to use GarageBand for techies who don't have the time:

1. The key thing to understand is that there is the concept of tracks, you get one by clicking in the upper left and you can create New Track. Each track has a set of attributes which you get by double clicking on the the pane in the middle and then at the bottom of the screen you will see the property page. It has three entries called Track, Master and Compare.
2. The main idea here is that each track goes through a processing pipeline. So when you click on Track, you are getting all the pipeline pieces that are being passed. There is a set of "quick" button in the table like volume and so forth, but they are only hot links to the junk below. You will see there are two kinds of things, one is called Recording Settings, you can think of these as the base features of GarageBand, but the second is label Plugin's and these are the variable number of little "boxes" you can add to the sound flow. This is a metaphor for how it worked in the 1970s, people would actually put boxes in the analog signal path and this mimics that.
3. So the way this works is that in the Plugins section, you will see a variable list of plugins. They will turn bright blue is the pipeline element is enabled. So for instance, there might be Compressor and. Channel EQ. Depending on what track you select, the plug ins and the Record settings change dynamically. So if you just want to see things at zero, click on each of the little boxes (I know, the UI is weird) and open a popup with that property.
4. Now on the right if you do not want to be that analytic, are exactly the same controls in 1970s analog form. So if you see EQ, and click the button up, this is the same as clicking on channel EQ in the plug in list where you can drag and drop.
5. The final thing to understand is that there is a "Master" settings which is the other tab, this applies to *all* tracks and it has it's own settings. So many times when you can't figure out what is going on, it is Master that is mis set.

Finally, as an aside, I find that all the of the sounds that you see on the left are pretty useless for podcasts, there are collection of built in sounds and presets, like "Classic Voice". I've tried them all and they sound pretty awful, so I get the best results just with a good microphone (go Rode NT-1A!) and leaving everything flat.

The second thing to understand that many people will need is how to do fades. That is if you have some music and you want it to fade out. This is super non-intuitive, but the base interface in the upper left lets you do things like split a track and then delete it (like iMovie). Here Command-T is your friend, it is the equivalent of right click and split track. So you can get rid of things.

But the confusing thing is that the control to change and fade sounds is modal. That is you have to go to Mix > Show Automation and the behavior of the upper left completely changes. Now when you click there, you will get a point which sets the volume. To add another point, click somewhere else. You can drag and move these around to get the face curve that you want.

And in this mode, you can't select or copy things around, so remember automation means fade.

The final confusing this is how to do a Save As because Save As just gives you GarageBand files. Instead you have to go to Share > Export Song to Disk to see the way to get an MP3 file or something like that. And as a confusing thing, by Shareing here, the file is backed up in iCloud > GarageBand for MacOS. So look for it there when you upload if (for free) with Anchor.fm

So there you have it GarageBand for Podcasters in a few minutes. I'm sure there are easier products to use, but It is free and if I ever want to do orchestral stuff, I have it. Others use Audacity I know, so I have to give that a try sometime.