A very simple lesson

Shkamon mzee.
This is a greeting to an older person. It is respectful.
The reply is
Marhaba: the 'h' is pronounced in an Arabic way, at the back of the throat, a glottal 'h'.
You could also say 
hu jambo. More or less 'how's things?'

Habari - How are you. Also Yako. The reply might be Nzuri.

Salaam is another general greeting. It sounds Arabic because the Arabs traded the coastline of Africa for a long time.

Habari asubuhi - good morning. Pronounce the second word asuboy.
Habari za mchana - good afternoon.
Habari za jioni - good evening.

In the street a white person may be greeted:

Mzungu. Or if there are two or more of you, Wa Mzungu.

For the young ones, you could say
Mambo? Something like How's it going?
The reply may be Poa (pronounced almost like poor): it means something like I'm cool.

Karibuna is the plural of Karibu: they both mean welcom.

And if someone knocks at the door - you say Hodi?