You've got an empty file. Now let's add some recordings. There are a few ways to do this, but basically either File > Import > Audio File..., or more likely, drag and drop an mp3 or .wav into a track. (If you drag it onto the grey area below the tracks, it will create a new track.)
After you've added the tracks, double click on the names of each track and give them sensible names.
You'll probably also want to create other tracks for any other sources of audio you're going to include: music, clips, sound FX, narration, etc. Click on the + button to do this (or Track > New Tracks...). Again, give them sensible names.
Now align them. If they came out of Cast, they should be aligned already. Otherwise, you should have done some claps, and you'll find visible spikes which you'll want to line up by dragging the audio regions left or right until all three line up. (You can zoom in by "pinching out" on your trackpad, or by using the left-right slider in the upper-right-hand corner.)
OPTIONAL: If your recording is many sections, and especially if you have time codes for each section already (if you were jotting these down as you were recording), you might now go to each segment and split them up. When editing It's Just A Show, I often do this, and colour-code each segment, and then work on one chunk at a time. You can colour code things by going to View > Show Colors (or pressing opt-C). Select regions, and select a colour swatch in order to change the colour of those regions. (When editing, I sometimes choose a darker shade of a colour to indicate a region that I want to return to later, either to fix a flub or because I'm thinking of deleting a long section starting here, but I have to listen ahead to decide whether that's something which can be deleted. Or maybe if there's something I want to look up for a link or to fact check. Etc. Colours are useful for larger editing projects.)
OPTIONAL: If you press the F key (or click on the film+camera+music icon on the toolbar) you'll open the file viewer panel. Click on Media and you'll see a folder viewer. If you have a folder with music and bumpers and the like, you can drag it to the top, and it will be easy to access. These folders persist across editing projects, though. Here you'll see my IJAS folders. Anyway, add your folder if you'd like. Press F again to close that panel.