How do you say you're welcome my friend in Arabic?

In English we say hello but in Arabic we say asalam wa alikum.

There are actually a variety of ways to say "hello" in Arabic, depending on the country.

Salaam 3alaykum (سلام عليكم) is the most formal of these and can be used anywhere. The response is Wa-3alaykum as-salaam (وعليكم السلام).

Merhaban (مرحبا) is more casual, but still relatively formal and has more of a "welcome-vibe" to it. It is used mostly in the Levant and Iraq.

Ahlan (أهلا) or Ahlan Wa-Sahlan (أهلا وسهلا) are much more casual and are used almost everywhere.

Another answer:

Hello in Arabic is (marhaba), or in modern Arabic (marhaban).