Hi: I always wonder which is correct:

But it is a consonant: which is correct: A or AN?

Established practice is to go by how you would pronounce the acronym if you were reading it aloud; in this case, you would spell it out, so you would use 'an'. Of course, some acronyms can be pronounced as words, so you would write 'a Nato (or NATO, depending on the style you adopt) exercise' but 'an NGO'.