Photo editing is a vast subject that cannot be explained in one such thread.
A. What you do with the photo depends on the mode you have shot it and what are you going to use the image for.
People take pictures in JPEG/PNG mode commonly. Professional take in RAW format. The file size of a picture varies in ascending order from GIF, JPEG, PNG, RAW and so do the outcome of editing. Pictures taken or stored in GIF format cannot be edited very well.
If you are going to upload the image on the net, then you can edit, sharpen etc but if you are going to publish the image in a magazine, then it would be converted to CMYK format. Most experts do not sharpen the image in the camera or on the photo editor but do it after conversion to CMYK format. So, what you do depends on what you have and the final indented use of the image.
B. The quality of the editing, of course, changes from the various free software that you get and the professional adobe photo editor that wildlife and photography experts use.
Adjusting the pixels, resolution and resize need to done alway bearing in mind that the resultant image with not be of the same quality of the original JPEG or PNG image.
C. As far as pixels are concerned, it is the tiny unit that carries one of the basic colors (Red, Green, Yellow). Most images are set on the standard format ration for optimal viewing. For instance, the common image we are all used to is the 4inchx6 inch picture. If this is printed at 300 ppi (300 pixels per inch) that gives a good quality print then the image will have 1200x1800 pixels
The common settings that are used based on image size are as follows. You can use these settings. Please read it as Image size in inches followed by pixels for a good image and pixels for a high-quality image
5x7 image-900x1260 pixels-1500x2100 pixels
8x10image 1440x1800 pixels-2400x3000 pixels
D. The image size ratio that most people use is 4:3, 16:9 and 21:9.
E. The practical way to learn photo editing and image re-sizing and adjusting the pixel resolution is by trial and error. Select a good image and then try the above and view the final image on the computer and an actual print out, this will help you to get the hang of it.
F. As far as cropping, sharpening, color, light change etc goes, for beginners, it is advisable to undergo editing rather than overdoing it. Please remember that the final image will look different on the computer screen and the mobile screen( images on mobiles look more sharp, perky and with a touch of little more color).
Among the many photo editors, PhotoScape is a good free download tool for beginners to get good images. Here you can edit the image and resize in standard acceptable options that the experts use.
G. Lastly it is useful to know that even a good image added to a power point, word file etc will suffer some loss of quality if we choose the 'compress image' option finally to save disk or storage space.