In Arabic grammar, the default harakaat that goes on all words is dumma (ُ or ٌ, the “oo” or “oon” sound). When you don’t know the vowel, apply dumma. Other harakaat are for other cases–for example, posessive case takes kasra (ِ or ٍ, the “ee” or “een” sound).

Harakaat are two types–short (aa, ee, and oo) and long (aan, een, and oon).

The second point to remember is definitivity. A definitive object means a specific object, not just any old object. For example, the word kitaabun (كِتَابٌ) means “a book”, while al-kitaabu (الكِتَابُ) means “the book”. Al-kitaabu is definite, kitaabun is not definite.

The rule is that definitive words don’t take tanween. They always take the single harakaat. So if you want to turn “a camel” (جَمَلٌ) “jamaloon” into “the camel”, it becomes al-jamalu (الجَمَلُ). The same applies to all nouns (and to adjectives).

You can find an excellent series of short posts on Arabic harakaat (with gigantic font-sizes) at Islamic Forum.