change file permission

[PeterHanssen]

I want to upload some files to my homepage. I tried to do that in my ftp location. I get a message that I am not allowed to make changes in this directory. I tried to change the file permission (it is set to 755 and I want to change it to 666 or 777). Whatever I try I can't upload and I get each time the same message that I am not allowed to do this. I have the following questions:
-Which permission should I change: directory public_html or the file I want to change.
-What permissionnumber is the best to use?

I work with Apple Mac. OSX.4.6

Thanks for answering

Peter