One easy way of doing it would be with the magerun tool. If you download and install this following the installation section of the readme, one of the commands it provides is the ability to create an admin user from command line.
$ n98-magerun.phar admin:user:create [username] [email] [password] [firstname] [lastname] [role]
Do you know if I can also do it via the phpmyamdin page ?
Maybe with SQL ?
Because I dont know how to install Magerun correctly
For Mysql scripts you can check these links: