It's pronounced "ko-oo".
It's important to note that Hawaiian grammar is slightly different from English. "Ko'u" can be used for both singular and plural possessions.
For example:
* "My car" would be "ko'u kaʻa"
* "My friends" would be "ko'u mau hoa"
It's also important to remember that in Hawaiian, the possessive comes after the noun, unlike in English where it comes before.