File owner apache or directadmin

Hello, first of all my english not perfect!

I m using joomla. when i was using cpanel not problem in joomla file/folder permissions.
Now i m using directadmin
file permissions 755 but folders unwritable in joomla

what can i do?
can i use chown? how? i dont know using.
if chown command not enough what can i do?
thanks