There's no written documentation or built-in tutorial mode for BrickStore, but most UI elements have tool-tips: just hover your mouse pointer over an element and you'll get a short description.

In addition, there's a bunch of videos out on YouTube that should help you out.

With new videos popping up all the time, it's hard to keep this list up to date though. Your best bet for helpful videos that aren't completely outdated, search for brickstore on YouTube, click Filter and sort by Upload date.

Here's a convenient link for that:

That being said, here's a small collection of channels and videos that showcase recent versions of BrickStore:

GreatBrickLab has a bunch of tutorial videos:

JustABrickInTheBucket has a series of quick tips:

Here's also a few links for German speaking users: