Crop tool

Louis, I think the simple answer to your question is that you should select the Crop tool and then hit "Clear" on the Option Bar, but I'm going to tell you a different way to accomplish what you're trying to do. I happen to like the way I do it better than using the Crop tool, but you can learn about both and then decide for yourself which you're more comfortable with.

With your image open on your desktop, select the Rectangular Marquee tool. Then, in the Option bar (the one just below the main menu) you'll see a box where it says "Style". It'll default to Normal, but there are a couple of little arrows on the right of that box. Click on the arrow and select "Fixed Size" from the drop-down list. Next, go one space further to the right and type in the Width you want your cropped photo to be; then type in the Height in the last box to the right. (The default size is in pixels, but if you put in the number '4', for example, it automatically begins to measure in inches.)

Now - still having the Rectangular Marquee tool selected - click anywhere in your image. You'll get a box bounded by those famous marching ants, and that box will be the exact size you chose in the Option Bar. You can click and hold anywhere within the area of the ants and move the box around until it's surrounding the part of the image you want to keep. Once you've got it where you want it, go to Image>Crop, and your picture will be resized to your exact specifications.

A lot of people swear by the Crop tool, and if you're comfortable with it, it works very well. I just think it's too bossy. :) I like having more flexibility, and I also like not having to worry about whether or not any resampling might occur if I forget to check the settings before I use it. The Marquee tools also allow you to make oval selections.