Replacing existing site authentication with WordPress

Hello all, I’m guessing this is not a new inquiry about this, but I have a simple site that users log into. The site is just for reporting some sports stats.

The login is very crude and I would like to switch the frontend to WordPress. In doing that, I would like to move my authentication from the existing site to WordPress user authentication.

Is there a tutorial anywhere for how to do this? I have some defined user levels but they are very similar to the WordPress Author/Admin/Editor levels. If I could just switch to that, it would be perfect.

Thank you!