Ok I am a total rookie at birthday cakes and decorating. (basically i've made your standard 9x13 box cake prior to this) I am wanting to do a double layer 9x13 sheet cake for my son's bday and could use any and all tips. My two main ?s are how do i level the layers and what is the best way to smooth the icing? Thanks

To level I would try to prevent the hump in the first place.  Several ways to do this.  Bake at 325 instead of 350.  Put one or two metal flower nails in the batter before baking.  Spray them with some Pam and put them in upside down.  This helps conduct heat into the center of the cake.  Third is to use magic strips.  You can buy them or make them.  This helps to insulate the outside of the pan and slow the baking on the outside so the outside will raise higher.  You can make the strips by using an old bath towel.  To smooth the icing you can first do a crumb coat.  That is a really think layer of icing to seal in the crumbs.  With this layer you will still see the cake through it.  Let that set a bit then put on a  LOT of icing and spread it around.  Then take your spatula and/or bakers blade and gradually using long strokes remove the excess icing, a little at a time.  Some ways to get more info on all of these things is to use google.  Just type in the topic followed by cake central and you will get more threads.  For example.....       smoothing buttercream cake central            and additional helpful threads will come up.  My favorite youtube decorators each have great tutorials, including smoothing.  They are tonedna1 and seriouscakes.