Hi garfield,
The gook that Tom mentions (by Katrin Eismann) is a VERy good book, though you say you don't have time to read a book (being a student). But you should have summer vacation, right?
Short of reading a book, I would start by perusing some of the challenges, pick one that looks interesting to you, then read the descriptions that are included with the entries. You will learn a LOT about Photoshop just by learning how people apply it to the challenges. You can also peruse the rest of these forums (esp. the tips and tutorials area) and pick up a LOT of really useful information. And, as Tom said, if you have an image that you are working on and having specific trouble, then post here and you'll get help pretty quickly. We really do like to help, but it's a lot easier when someone gives us a specific problem to solve rather than a general question.
Now - to try to answer your specific questions:
How can you tell that the image or picture that you have is already perfect or that no more enhancements can be applied to it that will make it look worse.
This is a difficult question to answer, because there could be so many things wrong with a photo. The truth is that this "wisdom" will come with experience - as you work on more and more images, you'll start to realize which problems you are able to improve and which ones will give only minimal improvement for the amount of work you'll have to put into it. In general, I usually start looking at the different channels in a digital image (RGB, CMYK and Lab) to see exactly what kind of information exists for me to work with. I also look at the levels (histogram). In general, out-of-focus images are impossible to make in-focus, though there are techniques (one is
here) which will trick the eye into thinking the image is more in focus than it really is. Also, very noisy images can be improved, but it's very difficult to remove all the noise without affecting the integrity of the image, so it becomes a balance that you will learn as you work with those kinds of images. Some photos are so faded that there just isn't any information left to build up (and recreate) the image. It is impossible to restore data that doesn't exist any more, so you have to decide if you can "paint" the information back in and still have it look like a photo - or if the result will have a "painterly" look - and is that acceptable? As you can see, this is a very complex question. Again, looking at some of the challenges on this site will give you a feel for what is possible for various problems.
What is the best image format and how big is it at its most? like in jpg can it reach the size of 1 mb?
The best image format depends on how the image will be used. Check
here for a discussion of file formats that apply to retouching.
As far as the size, it really depends on the size of the image. Obviously, a 100x200-pixel image will have a much smaller file size than 1000x2000-pixels. For a given image size, the various file formats will produce different file sizes. JPG tends to be the smallest, but that is because it uses a compression algorithm that actually eliminates some information that is kept in other files sizes. And if you have a large image that is saved as a high-quality JPG, it is easy for the file size to be significantly greater than 1MB.
what is the difference of digital image from normal image from screenshots?
I'm not sure I understand your question. What is your definition of a "normal image"?
btw, what's the limit of the words in a single post?
I've never run into a word count limit (and some of my posts can be pretty darn long.

)
Jeanie