iPhoto makes cropping very easy. I'm sure you can do it without any help from me. But here is a little trick you might not have run into:
rotating an image. Rather, you are changing the orientation of the image.
The cropping tool has a dropdown box that shows different size ratios of photos. If you drop down that box, the last two items are Constrain as Landscape and Constrain as Portrait. Chances are, those two options are greyed out.
The trick is to select a standard photo size (for example 4x6 or 5x7) to constrain to. That makes the options to constrain to landscape or portrait become available. Select the option that will change the orientation of the photo. Then you can crop the image at that orientation.