быть+noun in the instrumental case, always: Я буду врачом - correct Я буду врач - incorrect быть+adjective in its SHORT form OR быть+adjective in its LONG form+noun: Я буду счастлива - correct, the short form of the adjective is used because there's no noun after it Я буду счастливой женой - correct, the long form of the adjective before a noun is used Я буду счастливая - grammatical but less used Я буду счастливая жена - incorrect because we have a noun after the adjective here More about short and long adjectives in Russian http://masterrussian.com/aa041601a.shtml

You use the instrumental case with the copula быть in the past and the future if a noun is a part of the predicative expression (https://en.wikipedia.org/wiki/Secondary_predicate). Grammatically, both the nominative and instrumental are possible (as usual, a lot depends on the context), but in the modern Russian there is a strong preference for the instrumental case, especially in the future tense.

я буду счастливАЯ - a woman said я буду счастливЫМ - a man said я буду ВРАЧ - this is the nominative case which is can't be used here я буду врачОМ - and this is the instrumental case which is correct