To get the correct aspect ratio in Photoshop, choose the "Rectangular Marquee" tool and in the top menu, set the "Style" to "Fixed Ratio" from the drop-down menu, and enter the dimensions. For example, Width: 3, Height: 2. Draw your rectangle around where you want to crop, center it correctly, then click "Image" --> "Crop". Finally, "Image" --> "Image Size" and select your pixel width (i.e. 1600, 1200, 1024, etc), and since you set the ratio, the height will be automatically adjusted.
As to your image's quality, I'm still a novice, but the overall quality is not there, either due to a large crop, or the camera quality. I'm not sure this one is salvageable. However, I will say that you are doing the right thing by asking for help and reading as much as you can. The Editing Guide and Rejection Guide are extremely useful for first-time uploaders, but it will take some time to get the hang of it. Don't get discouraged! It usually takes people quite a few tries before they get their first acceptance. Good luck!