All Arabic letters are used .But about the words I put here link to a page contains Vocabulary List of most used words http://arabic.speak7.com/arabic_vocabulary_adjectives.htm Hope that helps

I would probably think words like: with = ma3a = مع between = bayna = بين you know, those preopositions that complete the meaning of sentences but I guess it depends on the subject you'll be typing about too but if you are asking about a letter, surely "alef" with its different forms أ إ ا & "al" as it's attached to the beginning of words to give the meaning of "the" ال like ال + ولد الولد = the boy and of course "waw" و meaning "and" sorry for the long reply, its kinda like an open ?

I think there are some letters used more than others in Arabic like: m"م" , n "ن" , a "أ", r "ر", o "و" and s"س". Most used words are the pronouns like : I= أنا "ana" , you = أنت "anta" , she =هي "hia" and he = هو "hoa".