As I recall, Hesham, there is no rule for the CH. ;-)
Modern English came from many different languages, not just old English, and since many English words have been influenced by Latin, German, Old English, Spanish, French, etc, that's why letters aren't uniformly pronounced the same all the time as they are in other languages.
You'll just have to learn them with each word you learn.
I've been going through the same thing learning Irish, so don't feel bad. ^_^.