Yes Leif is right.
It is very powerfull software, in my opinion much better than Adobe Photoshop. I would reccommend paying a visit to the plugins page. And then try out some tutorials. If you dont like it you can just un-install.
Use the rectangle select tool to select the part of the image you want to keep.
Go to Image>Crop to selection.
A bit of advice: Maybe you should play around with PND, ya know, experiment a bit.