To login to Behance, you must use an Adobe ID.
If you have an Adobe ID already but no Behance account:
Simply login to Behance.net with your Adobe ID to create a new account. If you're already logged into your Adobe ID via another site, you can simply visit Behance.net and click "Activate!"
If you have an Adobe ID and a Behance account already:
Simply click "Login," then enter the Adobe ID and password that's associated with your Behance account.
Not sure what Adobe ID is connected to your Behance account? More help here.