With asking this question I firstly wonder if the person is: A) serious, b) joking, or c) uncertain, so the best stance is to answer it from a polite blend of all three.
It's also important that I mention what my own denomination/beliefs are - so to place my own cards on the table as such. Mine are new everyday, and that's what I would say if asked on the street by anyone.
I see the answer as something simple, which is actually a question, which is: 'What do you believe in instinctively?' I believe in a lot of things instinctively. My work, my relationships, my passions etc.
I see a term like 'God' as a synonym for many things. My own personal synonyms on this matter relate to things like writing, friends, art, environment... I see all of them as gods, goddesses, divine entities, science, life etc, and I don't have to think about 'how' to 'believe' in them, it just happens when I respect and work with them.
I feel that there are no differences in the correctness of the passions anyone follows. To believe in something a great deal is to give the ideology strength, and this can be anything.
So the answer is basically: Within your evolving drives, endevours, and accomplishments; once you understand how you believe in them, you will be closer to answering your own question.