Hello: Marhaban.
Let's look at it in a dialogue.
Ali: Marhaban, Amir.
Amir: Marhaban, Ali.
In Arabic, to say "hello", you say "marhaban". To make this more clear, let's take a look at some more examples:
Fatimah: Marhaban, Amir, Marhaban Ali.
Ali: Marhaban, Fatimah.
Amir: Marhaban, Fatimah. Kaifa haluki?
"Kaifa haluki?" means "How are you", but don't worry about that quite yet. For now, focus on "marhaban".