It’s all done for you (thanks Rob 
)
http://wiki.freepbx.org/display/HTGS/Installing+FreePBX+13+on+Debian+8.1
pay PARTIULAR attention to the bit that says
. . . .
As part of this install, you will be asked several times for a mysql password. You can leave this blank (just push enter) as the instructions further down will generate a secure password. If you set a password now, you will cause problems further down. Please do not set a mysql password unless you are confident in your abilities to secure a SQL server.
. . .
Are you so confident? If you are, please read
./install --help
before you do the
./install -n
bit.