I am a self taught baker love to make birthday cakes now I have been asked to make a wedding cake for september. Could you please help me to get the sides of my cakes nice and smooth before icing.  Do I start of with a big cake and cut smaller ?  
I addendum to what Jennifer S. has submitted:  You can trim the cake to level out any large deformations but filling it in with buttercream is probably a better bet.  Make sure to brush away any crumbs so they don't get trapped in your final layer of frosting.  The crumb coat technique is very helpful especially if you chill it before applying the final layer of frosting.  One trick that I've seen used is to warm a thin, metal bench scraper in hot water, wipe it dry and lightly smooth over the buttercream.  The heat helps melt the butter slightly and create a really smooth finish before applying decorative icing.  Craftsy.com provides a free instructional on what I just described. 