In you master page (layout or pages) you should specify the using of UTF-8 like in the next example:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Then in your web.config file you should have the next settings for specifying encoding for Arabic:
<globalization fileencoding="windows-1256">
requestEncoding="windows-1256" responseEncoding="windows-1256"/></globalization>