if i were you i would choose past tense, because i know that if we talk something about what we made today, we should past simple. because today indicates us a specific time. we use present perfect when we don't a specific time. so it should be tell me everything about your exam (which) you had today.
and what if we add sounds better because you're telling us at the moment.
for example you drank alcohol unbeknown your father. then you say him what if i drank alcohol, what's to you? ( certainly, i don't offer like this dialogue with your father, it is very rude)
on the other hand, you didn't drink alcohol, but you have an intention about drink alcohol, but you aren't allowed to drink by your father. so you say; what if i drink alcohol, nothing will be.