I am wondering if I were to make a cake with fondant and buttercream covering, do I leave it outside or do I put it in the fridge. I have heard that you cannot put fondant in the fridge as it ruins it.

Leaving it outside depends on the filling you used - does that need to be refridgerated? You can put the cake in the fridge, and it will have condensation after you take it out, but if you leave it alone to dry for a few hours - no touching - it will be OK. HTH