"If your device has lifetime maps, you can simply just download Garmin Express. You can skip the registering step. There is both a Mac and Windows version. Simply connect the device, and you should see the logo on the Garmin device change to notify you its connected.
However, if you need to purchase maps, you will need to register and create an account and pay for the map updates first."
If your Garmin GPS has an SD card, which most have, then use OpenStreetMaps. Just google 'OSM for Garmin', you will find that you can download weekly updated maps for most any country in the world for free. Garmin Topo maps use the same OSM maps but make you pay for them.