AI've tried to smooth my buttercream before putting on fondant but my cakes always look so bumpy?. Am I supposed to put bare minimum amount of buttercream under the fondant or do I put a good amount on.. I just want a nice smooth cake

Look into using ganache under fondant instead of buttercream.  I tried it and never looked back!  It makes a nice, hard, smooth surface for the fondant. No squishy buttercream moving around under the fondant.

AThat's a great idea unless they don't want chocolate, I'll give it a try

Um... do you have a smoother?

When you lay the fondant, be sure there are no air bubbles....if you see air bubbles, poke them with a pin and release air, continue smoothing. 

GREAT idea with Ganache.....I would like to try that!!

AI do have a smoother, I have 2. I tried them and even put crisco on to try to get them to glide better ( bad idea) and the air bubbles weren't a problem my cake just look lumpy, I wish I could attach a picture

AIs frosting underneath smooth? Too thick?

it sounds like your buttercream is buckling under the fondant. especially if you aren't using a crusting buttercream, I would try chilling the cake first.