Fortunately, Garmin uses a common file to store your data, known as a .fit file. These files can normally be uploaded to the site of your choosing, Strava included. Here’s the high level overview of what you need to do:
1) Hook up your Garmin device to your computer using a USB cable (most likely the charging cord that came with your device).
2) Navigate to the device in your file explorer and save the .fit file locally (you might be able to upload it directly from here, but I like to have a backup on my computer anyways).
3) Find your site’s upload instructions. For Strava, you need to login through a web browser (if you’re on a mobile, you may need to request the Desktop site).
In the upper right of the screen, click on the plus (+) sign, then on Upload Activity.
Click on File.
Then on Choose Files.
Select your .fit file, and voila! You’re all set!