"Assalamu alaykum"

Okay, there are several different ways of spelling this. I personally like the way I used above. I'll list a few other ways of spelling it.

1. As-salamu alaykum

2. As-salalmu alaikum 

3. Assalamu alaikum

4. Assalamualaikum

5. Assalamualaykum

6. Asalamu alaikum 

7. Asalamu alaykum

I am sure that there are many more too.

Anyways, this is a very commonly used phrase. It means "May peace be upon you" or "Peace be upon you". This is pretty much a replacement for "Hello".