You need a form of logic where you can accept that the existence of bananas is not a paradox - but that we can't trace their origin back further than shortly after the big bang. You have to be able to admit that we don't yet know, and possibly may never know.
Hint: start with what we do know, and work from that. Don't start with infinities and try to work back - infinities are very complicated and difficult to work with, particularly in metaphysics. If you'd like to know how the universe could arise from nothing, Laurence Krauss has
a good explanation. If you want to discuss where the nothing came from, that's a whole other thread (the Hackenthorpe Vacuum - take an ordinary vacuum, and suck all the vacuum out

).