How do I use Garageband on my iMac?

Start here => GarageBand - Official Apple Support

If you haven't use GarageBand, or any sort of A/V editing software, you will have a bit of learning to do. YouTube has plenty of (free) tutorials which will start you off. After that it's a matter of building your knowledge by using the software.

If you have used A/V editing software GarageBand has skills in common with those which you can apply.

Either way, dive in and try it. You can't break anything.

Which system version is running on your Mac? And which version of GarageBand do you have installed? Do you already have GarageBand installed or are you having trouble to find a version that will run on your Mac and system version?