1- How to Say “Hi”?
What are the most common sentences or expressions, when you see a person for the first time or want to start a conversation?
1-1- Islamic greeting; السلام عليكم (و علیکم السلام), you may hear it in most of Arabic countries, specially in Iraq and Saudi Arabia.
We can also say سلام علیکم instead of السلام علیکم
1-2- Normal and common greeting; أهلاً و سهلاً  (أهلا بك)which means “regards”, or you may say “مرحبة” (مراحب) , “greetings”.
2- How to say “How Are You?”
1-3- The most common expression is: کیف الحال؟  means: How is it going? or  کیف أنت؟ means “How are you?”
1-4- For replying we could use the following sentences: الحمدالله  (Thanks God) – شکراً (Thanks) – أنا بخیر (I’m doing well, I’m ok)
3- Other Common Expressions
تحية مباركة
تحية طيبة
تحياتي واشواقي
4- How to say “Goodbye”?
a. Islamic greeting; السلام عليكم
b. Normal and common greeting;  إلی اللقاء > I wish I see you again. في أمان الله means: God may protect you.