rules for na'vi phonetics can be found in this very old but timeless post: http://languagelog.ldc.upenn.edu/nll/?p=1977
abide by those rules when loaning words from English. the typical thing people do here is use the na'vi word of the pandoran animal which most closely resembles the terran animal.
also popular are the unofficial words for dog and cat:
nantangtsyìp = dog
palulukantsyìp = cat, palutsyìp for short.
not official, but if you see them, thats what they mean.
someome explain the Na'vi name formula, i dont have time atm