i want to make a box cake covered in fondant. any tips on how to get sharp edges rather than rounded edges?
also any advice on how to do the lid of the box?

Don't know how much time you've got, but Sugarshack has a great video out on fondant where she uses two fondant smoothers and pushes the fondant into a sharp edge after she trims and crumbcoats the cake. Really great to watch and learn.

I refrigerate my frosted cake until the buttercream is firm, and then use a warm spatula to make those edges super-sharp. Then once the fondant goes on you don't need to do much manipulating to get sharp fondant edges as well. As suggested, two fondant smoothers is a great way to make the corners perfect.