a 403 error message is usually a permissions error.
there could be multiple reasons you see this message.
the cause is usually
1) Incorrect file or folder permissions
2) Incorrect settings in the .htaccess file.
As a reference, generally files get a chmod (permission of) 644
while folders usually get chmod (permission of) 755
locating the cause could be more complicated than that though.
since it could be your htaccess file, could be the index file,
or some other file you try to access like maybe a login page.
providing the link to the page where you see the error might be helpful.