Greetings according to the time of the day:
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A second reading (by Muhammad Nor Ismat, a native speaker)
Selamat pagi (=Good morning)
Selamat tengah hari (=Good afternoon - from noon to 2p.m. or so)
Selamat petang (=Good afternoon/Good evening)
Selamat malam (=Good night)
Other greetings:
Selamat datang (=Welcome)
Apa khabar? (=How are you?)
Khabar baik, terima kasih.(=I'm fine, thank you.)

For those who want to know more:
Note: The following rules are not at all rigid. They are only meant to be used as a guide:
From sunrise (about 5 or 6 a.m.) to 12 noon, you say Selamat pagi.
Between 12 noon and 2 p.m. or so, you say Selamat tengah hari.
From 2 p.m. until sunset (about 8 or 9 p.m.) , you say Selamat petang.
From sunset to midnight, you can say Selamat petang or Selamat malam (depending on whether you are arriving at an evening function or leaving).
But after midnight and until sunrise you can either say (considering that it is already a.m.) Selamat pagi or, if you are taking leave of your colleagues after a night shift Selamat malam.
A word that does not go by the clock hour is siang hari which means daytime i.e. when you don't need to turn on the lights.
If all this sounds too complicating to you a passe-partout (or all-purpose) greeting which can be used at any time of the day is Apa khabar? It's equivalent to "How are you?" in English or "Qué tal?" in Spanish. The standard answer is Khabar baik, terima kasih. (=I'm fine, thank you.)
Incidentally if someone says Terima kasih (=Thank you) to you the standard reply is Sama-sama (=You're welcome). An Indonesian though would reply Kembali instead of Sama-sama but that's another story.
Another useful expression to learn is Maafkan saya when you wish to apologise for something. Instead of this you could also say Minta maaf or Harap maaf (= Excuse me). The usual reply to this would be Tak apa (=It doesn't matter) or Tak apalah (see Lesson 20). Note that the suffix lah is always added to soften the tone or to make something less formal. ("Yeslah it's true" I can hear your Malaysian friends saying!)
Note also that greetings in Malay are normally followed by the name of the person to whom we greet. Thus if we are greeting Mr. Ahmad, we don't just say Selamat pagi but Selamat pagi, Encik Ahmad or Selamat pagi, Cik Ahmad.. Note that while in the written form "Cik" is equivalent to "Miss", in its spoken form "Encik" for Mr. is very often contracted to "Cik".
If we are greeting someone whose name we do not know (as when asking a stranger for information) we can use "encik" (mister) or "puan" (madam). Thus: Selamat pagi, encik or Selamat pagi, puan (for a married or elderly woman).
When addressing someone whom we know to be a teacher we can use cikgu eg. Apa khabar, cikgu?
By the way the Malay word for "teacher" is guru.
Example: Dia guru anak saya. (He/She is my son/daughter's teacher).
When addressing a titled person we can just use his title without his name. Thus Apa khabar, Datuk? or Selamat pagi, Tan Sri.
"Datuk" (sometimes spelt Dato or Dato'), which literally means grandfather, is a title conferred to "prominent" Malaysians (ah, I'm treading on dangerous ground here by using the word "prominent" since there have been cases of such titles being bought!) by the King or Sultans on their birthdays. The wife of a "Datuk" should be addressed as "Datin" but if it is a woman who has the title of "Datuk" in her own right (which is quite rare) her husband is not affected by her title.
Higher up on the title hierarchy is "Tan Sri" and even higher still is "Tun". The wife of a "Tan Sri", by the way, should be addressed as "Puan Sri" and the wife of a "Tun" as "Toh Puan".
Incidentally "Tun" is limited to 60 living holders at the moment (at the initial stage it was limited to only 5). If you like to read more on this subject click here.
When you know that besides the Yang di-Pertuan Agong (the King), each of the thirteen States also confers such titles every year you will not be surprised by their proliferation.
Go here (Honours List) if you wish to have an idea of the awards of the past few years.

WISHING AND HOPING:
First Wishing:
As you have seen above all wishes and greetings start with the word Selamat (followed by another word).
Other examples are:
Merry Christmas = Selamat Hari Krismas
Wish You a Happy New Year = Selamat Tahun Baru
Wish you success = Selamat berjaya
Or if you prefer you can also say Selamat maju jaya (the word maju meaning "progress").
There is also Selamat sejahtera (May peace be with you) and Selamat Hari Jadi (Happy Birthday)
Incidentally if you want to put the above wishes in a full sentence you can by starting with Saya mengucapkan... meaning "I wish you (success, etc)"
Thus the sentence "I wish you and your family a Happy New Year" would be translated in Malay as:
Saya mengucapkan Selamat Tahun Baru kepada anda dan keluarga anda.
But mengucapkan does not mean "to wish" but rather "to express" in this sentence:
Bila dia mendengar kematian kawannya dia pun mengucapkan takziah kepada keluarganya. (=When he heard about his friend's death he expressed his condolence to the family.)
Apart from Christmas and the international New Year on January 1, there are the following occasions when you might have to express your greetings to your Malaysian friends:
End of the fasting month by wishing the Muslims Selamat Hari Raya.
The Chinese New Year by wishing your Malaysian Chinese friends Selamat Tahun Baru Cina (unless you'd like to wish them in Chinese in which case you'd say "sin nien k'uai le" or "kong si fa ch'ai" the official spelling being "Gong Xi Fa Cai").
The Indian New Year by wishing your Malaysian Indian friends Selamat Hari Deepavali.
Talking about the Chinese New Year here is an interesting article on the tradition of giving red packets (angpows) to children (and even 50- or 60-year-old adults if they're still unmarried)! In fact this age-old custom has been "adopted" by the Malays and Indians too so if you are staying in Malaysia for some time you might want to know more about it. Go here for an interesting background reading on the subject by KTemoc, a popular Malaysian blogger.

Next Hoping:
When you tell someone that you hope he will get a job, for example, you start with Semoga (can be replaced by Moga-moga or Mudah-mudahan). All three terms mean "It is hoped that..." or "I hope that..."
Thus Semoga anda dapat pekerjaan yang diminta itu means "I hope you will obtain the job you applied for" and
Mudah-mudahan saya dapat tolong anda means "Hopefully I'll be able to help you".
Note the two different meanings of the verb dapat in the above two sentences:
1. meaning "to get" or "to obtain" eg.
Dia dapat banyak hadiah untuk hari jadinya. (= He obtained many gifts for his birthday).
2. meaning "can" or "be able to" eg.
Dia tidak dapat membuat kerja itu. (= He is not able to do the job).
There is another way of saying the same thing (in fact this is closer to the English way) since the Malay verb for "to hope" is berharap. So you can also say:
Saya berharap anda akan dapat pekerjaan yang diminta itu ("I hope you will obtain the job you applied for".)
Other examples on how to use Semoga:
I hope that you will get well soon (or if you prefer "I wish you a speedy recovery") =
Semoga cepat sembuh or simply Harap cepat sembuh
I hope you will pass your exam = Semoga lulus dalam peperiksaan anda.
I hope you will be happy in your new life = Semoga anda berbahagia dalam kehidupan baru.
If you want to split hairs you can always say that Semoga berjaya means "I hope you will succeed" while Selamat berjaya means "I wish you success". But to all intents and purposes it comes to the same thing so in this particular case you might just as well use the first word that comes to your mind.
In the case of wishing someone good health though you have to use Semoga or Moga-moga or Mudah-mudahan but not Selamat.
Thus you would say Semoga sihat selalu meaning "I hope you will always be in good health".
Want some more? Well, sihat walafiat is of Arabic origin and means good health but is quite often used by the local Malays. Thus if you have a Malay friend you could write:
Semoga anda dan keluarga anda dalam keadaan sihat walafiat (= May this find you and your family in a state of good health).
And in case you are the religious type you might wish to say "I pray that.." In Malay it is Saya berdoa or Saya berdoa kepada Tuhan (I pray to God).
Example:
Saya berdoa semoga ibu anda sembuh dengan cepat. (I pray that your mother will have a quick recovery)