To begin the process of fixing suPHP permissions you'll need to ensure you have SSH access for the account you want to fix first.
This guide will help you to access the account on the server via SSH.
Permissions:
Files - 644
Directories - 755
The /public_html folder should be set to 750 at all times, and ensure this is not changed.
If the permissions are not correct you'll need to run the following command line:
chmod XXXX FILENAME/DIRECTORY
Replace XXXX with the permission number and FILENAME/FOLDER with the name of the file or folder you are changing permissions for.
Please ensure that your account is fully backed up before you change any permissions to prevent future issues.