how do I get a really smooth clean edge on a square buttercream cake? I cant seam to get the edges really straight and smooth.

also my buttercream cakes always have tiny little holes all over. how can i avoid these holes. there is no way i can do a pretty smooth buttercream cake. please help me stop hating buttercream cakes.

Try the upside down method for frosting. Use freezer paper instead of parchement. For the first layer of frosting on the paper make it very thin, freeze it, and follow with a thicker layer. This will cut down on the pockmarks.

never heard of the up side down method. how does this work

See these tutorials for help. There are lots of tutorials on this site on many subjects.
Frosting a square cake:
http://cakecentral.com/tutorial/how-to-frost-a-square-cake
Upside down frosting technique:
http://cakecentral.com/tutorial/upside-down-icing-technique-for-perfectly-smooth-icing

What do you mean by tiny holes? Are they air bubbles? If so you have to paddle your frosting with your spatula against the side of the bowl to burst them before you apply it.

I take two cake boards slightly bigger than my cake, and tape them together for strength. Then I cover them with freezer paper (shiny side up). Secure it on the back of the boards with tape so it won't slide around. Trace the outline of your cake pan onto the paper. Spread a thin layer of frosting onto the paper filling the outline and then place in the freezer to firm. Repeat with a thicker layer and place in the freezer to firm. After your cake is baked and cooled, turn it out onto a board covered with parcement paper. Then place your chilled frosting board on top of the cake, making sure the frosting covers all the cake to it's edges. Then flip the cake over. Trim any frozen frosting off the freezer paper board.so it is even with all the cake edges. Frost the sides of the cake. I use a hot spatula and then Viva paper towell to smooth after it's crusted. Return to freezer to allow to firm up. After it's firm cut the freezer paper lose with a box cutter in an outline around the cake. Then place your decorative cake board on the top of the cake and flip again. You should have a cake that has it's top covered in freezer paper exactly to all it's edges. Begin to slowly peel the paper back at a 45 degree angle. If it's still stciky, put it back in the greezer for a while. Try to peel again. Hopefully, you should have a smooth top and sharp edges. You can also google the method if my explanation is as clear as mud.