When I open a TeX file containing arabic letters, I only see weird letters. I tried WinEdt, Emacs, TeXMaker. But when I open the same file using Notepad, it looks OK. What shoud I do?
You need to use an editor that can handle and display Unicode-encoded characters. The ones you mention -- other than notepad -- are probably not Unicode-aware. Actually, the very latest version of Texmaker is Unicode-aware; you may be using a somewhat obsolete version of that program.
Try opening your file with TeXworks, TeXshop (if you use a Mac), or any other editor that's Unicode-aware, and you should be in business.