all files have 777 chmod - what are security risky

I am novice so that before installation I changed attributes of all files to 777 (I did not want problems in install process). But now after installation I dont know if some files with 777 atribute are (or not) security risky for my phorum. Please tell me to what files (*.php) I must change attribute (and what chmod must be) so that my installaed phorum be secure and functioned. Thank you.