What is the proper way to state this; ';I want a opossum.'; ';I want an opossum.';?

I would rather like an OpossumWhat is the proper way to state this; ';I want a opossum.'; ';I want an opossum.';?
any word beginning with a vowel should have 'an' instead of 'a'





and for certain words that start with H as well.