How do I make a square cake that looks like a box with straight lines. The cakes that I have done seem to have more rounded corners and the top of the cake has more of a 'mushroom' shape to the top of it. Any help would be great!

Trim the cake so it has that box shape!
I fill and stack, then shape the cake and crumb coat. Let it settle overnight, then trim again to remove any buldges and perfect the shape. Then cover in fondant.

Jodie

Sometimes, the corners will sag a little bit and cause rounded corners. I think you can try building up the corner just a tad with fondant before covering with fondant. Not sure how well it would work, but you could try.

I've seen where those who use ganache form their sharp corners with the ganache and then put the fondant on. It has to be a ganache that sets up firm.

I'm not 100% positive, but I think some people make fondant panels to get the look you are describing. I think that I read that somewhere around here...

CC Article on achieving square corners on buttercream cake:
(Also helpful prior to applying fondant.)

http://cakecentral.com/articles/109/how-to-frost-a-square-cake

Indydebi demonstrates frosting square corners:

http://www.cakecentral.com/cake-decorating-ftopicp-5396443-.html#5396443

HTH

Yes the SETTING ganache is a great tip - but also you need to have your fondant really thin - 1/8th of an inch or less - when your fondant is thicker you will end up with the rounded edges.

Use two smoothers to form a right-angle and this will also help to achieve your sharp edges