Hi. The only way to do this that I know of is in Mail > Preferences > General > Default email reader.
The preferences option may be disabled because you haven't set up an account in Mail yet. You would do that in System Preferences > Internet.
If you don't want to use mail and don't want to set up an account with your email address, you could create a google or yahoo account for this purpose only, and use it. Then select preferences and change the default email reader. Then disable the account you just created.
The only other mail app I've used regularly is Postbox, which worked well for me. The only reason I went back to Apple Mail was that my project management software has a Mail plug-in for associating each mail item with a file I'm working on. Postbox to my knowledge does not allow plug-ins.