The first time you run the server it should ask for your email/password that you would like to register; after that, it will ask for the same email/password that you registered as your login info.
Yup, there is no default password for a development instance.
If it doesn’t find an application database with a user account connected (either via a preexisting H2 metabase.db.mv.db file or env variables pointing it to a MySQL or PostgreSQL server), it will create a new application database and send you through the setup process.
We tend to create, delete and generally abuse these application databases during development without any major issues. Just delete that file and start again =)