Like you would say it in English. The o is similar to the vowel in English go, and the i is short as in hit. In vedic intonation, there was a stress on the o, stress has disappeared in many modern Indian languages, and in some Indian languages, all vowels are of the same short quantity. The y is also like in English, but as the last part of a conjunct in an initial position, it only colours the vowel slightly, and most people will not notice if you omit it.
Of course the y was vitally important in the history of the word! The j arose as a palatalization of the d in words like div, to shine, and dyuti, shine, (though the former has a strong day/sky/divine connotation) which are in turn cognate with Zeus, Jove, Tiw, and Tyr. The palatal vowel i/semi vowel y was responsible for the d to j change.