Q: How do you make dictation work when you have a microsoft keyboard and imac computer

Apple's instructions for using dictation are in Use your voice to enter text on your Mac - Apple Support. You would need to find what Microsoft uses as the FN key which may require contacting MS for support on their product.

Go to: Apple > System Preferences > Dictation & Speech and turn Dictation On.

From there you should also be able to either Re-set or Customize the Shortcut.

Windows key is can be mapped as Apple Key, otherwise keyboard is the same

You're welcome!