From the Wikipedia page on Ubuntu:
(/ʊˈbʊntuː/ (listen) uu-BUUN-too)
This is written in the International Phonetic Alphabet (IPA) and Wikipedia's English respelling system. The vowels are /ʊ/ uu as in FOOT and /uː/ oo as in GOOSE. The consonants /b, n, t/ are pronounced as they're written. The second syllable is emphasized. (The colon-like symbol after /u/ is just a length marker, not applicable for North American accents.)
The citation here is the official About Ubuntu page. I'm not sure what system
|oǒ'boǒntoō| is written in, but it might be the pre-IPA Oxford system, described here under the "POD" column.
Another pronunciation I've heard in North American English is /uˈbʌntu/ oo-BUN-too, with /ʌ/ as in STRUT. Another is /juˈbʌntu/ you-BUN-too, with the first syllable /ju/ = "you", which is arguably incorrect. The second syllable is emphasized in both of these.
While we're here, it's worth mentioning that the word "ubuntu" originally comes from the Bantu languages (e.g. Xhosa, Zulu). I've never heard this used to refer to the OS, but for context, Wikipedia has the Zulu pronunciation on the page Ubuntu philosophy:
This is way more complicated for an English speaker. The accents indicate tones, the /ɓ/ is an implosive, and the /tʼ/ is an ejective.