how do i get imovie on my computer

The version of iMovie that is available in the App Store requires a mac running macOS Catalina 10.15.6.

If you are running macOS High Sierra (10.13) as suggested by your profile then you will not be able to download iMovie.

Open your Safari browser, do not use Firefox and click on this link,

How to upgrade to macOS Catalina – Apple Support

Read Section 1 to see if your MacBook Air qualifies.

if it does then go to Section 4 and click on Get macOS Catalina.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

The App Store should say Get or Download, if it says‘Open’ then it is detecting

a previous download of the install app go to the Applications folder locate

Install macOS Catalina.app. Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates.