If you mean how do they mate:
The Na'vi are monogamous creatures who mate for life. The mechanics of reproduction are similar to that of humans and other Terran mammals. But their unique physiology provides the Na'vi with a level of intimacy unknown on Earth. Cultural anthropologists believe that when an appropriate mate has been selected (which can take many years), the male and female Na'vi will connect queues (called Tsahaylu) to create an emotional bond that lasts a lifetime. The intertwining of queues is both highly erotic and profoundly spiritual, but does not in itself lead to reproduction. Traditionally, once a Na'vi male has passed the tests on the path to manhood and has been accepted into the clan as an adult, he is not only allowed to make his bow from the wood of the Hometree, but he is also expected to choose his woman. After the woman has been chosen, the new couple are mated before Eywa.
That is taken straight from the Avatar wikia page Mating: http://james-camerons-avatar.wikia.com/wiki/Mating
but if you mean how do they reproduce, that is an ongoing discussion and is not exactly known.