How do you crop and resize - correctly?

First, WHY do you crop? Because you want to make the photo more pleasing to the eyes. A well cropped photo shows only what needs to be there.

Crop away sticking heads, hands, and toes, etc. Be the least forgiving of your photos; sometimes, it's the only--and most--effective way to constantly improve. Be your worst critic.

Second, HOW exactly do you crop correctly? Easypeasy. Remember that unless you want to change your rectangular photo to square, you should preserve its aspect ratio so it doesn't end up distorted. 

The following is applicable to Photoshop (although I've also used this on Microsoft Picture Manager during lazy days or when I'm rushing):

1. Hit the Crop button (simply press C in Photoshop) and select your entire image (not just the area you want to retain; we'll get there eventually).

2. Hold down the shift key then move the selected area according to your preference (note: while holding the shift button). This is to preserve the ratio of the image and prevent distortion.

3. Of course, hit the enter key to crop your photo. Told you it was a no-brainer.

RESIZING
Similarly, you can customize sizes (i.e., when you want to crop a rectangular photo to a square, or a 4288x3288 photo into 5x7 inches for printing, etc).

1. Input the sizes you want on the bar above your window in Photoshop indicating the size (e.g., 8 in for width and 8 in for height, for a square photo. Though you can also do it in mm, cm, etc.)

2. Recommended pixels per inch: 300 (especially if you're printing).

3. Hit the crop button, and when you select the image, you'll notice that the selection area will already have adjusted to your custom size. So crop away!

4. If you want to upload your photos (i.e., on Facebook, etc.), crop your photos to 6x4 inches or 7x5 inches, then save them as separate files so you still preserve your original photo size and simply delete your spare copies after. And yes, because Facebook compression sucks. That's it! :-)