How to download install and use streetmaps

I don’t want to make maps! I want to use them!!! I tried to export a map of my hiking area to my Garmin etrex20x. I cannot find the file on the etrex. How do I use this system? Is it compatible with the etrex20x? How do I know what I am exporting? I open the exported file on my computer and just get a bunch of weird lines of numbers and symbols.

Did you start reading around here:
And the FAQ? might be also useful as a one-step-at-a-time guide.