If I remember correctly, the first time I attempted to instal owncloud, I installed the LAMP stack, then configured MySQL with a root user and password, then I fired up a MySQL client and created a database, user, and password for owncloud. Then, the last part (that's where I am now), I went to my browser, pointed it to my server_ip_address/owncloud. That launched a wizard, where it asked for an admin user name, and a password, then the owncloud user, password and database name.
For the admin user name, and the admin password, I just put something in. I didn't need to go to my MySQL client to create it first. That's the part that's stumping me. Now, it's telling me that I have to put something in that already exists, which leads me to believe that when I deleted the LAMP stack, something got left behind. If I knew where, I'd delete it and start over.