This answer is based largely on my copy of The Klingon Dictionary (written by Marc Okrand, who created Klingon), which is reliable and good if you need a quick translation from English to Klingon or vice-versa. I'm going to cover most of the same ground as rotaredom's answer, just in more detail.
The book notes that there aren't really any greetings in Klingon; they're simply unnecessary. Klingon as a language is somewhat utilitarian in its application, and exchanges typically start with a straightforward statement or query by one party. Saying the Klingon equivalent of "How are you doing?" when you just want to figure out the new ship's departure time is simply wasteful. One answer to your question, then, would be to simply avoid a greeting. The less you talk, the less time you take up. That said, if you want to truly greet someone, you have two options. . .
Choice 1: Be utilitarian.
This utilitarianism is why all Klingon "greetings" are just introductory phrases. "nuqneH" is the one that seems to be commonly used, as rotaredom said. Its translation is "What do you want?" "Nuq" itself can be treated as an interrogative sentence on its own, simply meaning "What?"1. It can also be used as the start of a sentence, such as
nuq legh yaS
What does the officer see?
Additionally, "neH" is used as a form of "to want" (though it also used as an adverbial, to mean "only"). Therefore, we have the compound "nuqneH", properly an exclamation that stands by itself as a sentence.
Choice 2: Use an honorific.
The Dictionary notes that "neS" is the only honorific suffix in Klingon, used as part of larger words. For instance, "qaleghneS", coming from "qa" ("you"), "legh" ("see") and "neS", translates to
I am honored to see you.
This states a fact, and gets to the point. It may be used towards a superior, but this is not mandatory, and I don't think it's commonly used. If you wish to open the conversation with an inquiry, "HIja'neS" (which includes "HIja", normally used as an affirmative) can work. Its translation is
Do me the honor of telling me [about something].
Again, use this when asking a superior something, but only if you really want to be respectful.
Though I have yet to see any of the following used as a greeting, I imagine you could modify them to create some sort of opening remark.
- "bel" is the verb for "to be pleased", so when greeting someone, you could conceivably say "jIbel" ("I am pleased", with "jI" for first-person). Likewise, "Quch" is the verb for "to be happy". Using "jIbel" or "jiQuch" to indicate satisfaction upon meeting someone is a possibility.
- "maj" ("Good") or "majQa'" ("Well done") are exclamations used as praise. You could use these - preferably "majQa'" - when welcoming an inferior who has successfully completed a task, like blowing up an enemy spaceship.
- "toH" is another exclamation, meaning "Well!" or "So!" You would use this in, for instance, a sentence like
Well! I didn't expect to see you back alive.
1 Appending "Daq", to make "nuqDaq", yields "Where?"