how can I delete my mail app to free up space on my mac

If you are using a third party email app like Spike, Outlook or Spark, then deleting it is usually about as easy as dragging the app from the Applications folder and into the trash.

You might first check the website of the publisher for guidance as maybe there's an uninstaller to be used that might aid in getting any little bits that are elsewhere.

You should look at this: Free up storage space on your Mac - Apple Support

You can't delete Mail. In any case it only takes up around 28 MBs.