To sharpen a knife, you need to follow two main ideas:
24° - Hunting, Pocket and Sporting knives (THICK BLADES)
21° - Hunting, Pocket and Sporting knives (THIN BLADES)
18° - Kitchen, Fish and Meat knives (THICK BLADES)
15° - Kitchen, Fish and Meat knives; SCISSORS (THIN BLADES)
10° - Serrated knives
To check if the blade is sharp, use your nail, put the blade with a vertical angle on it (beware: don't push it! Just keep it there!)and see if it gets stuck there (= the blade is sharp) or if it slides away (= the blade is blunt).
In principle, sharpening a knife means removing metal from the blade's edge. You remove metal by grinding away the metal surface. Water helps making the abrasion caused by the movements of stone to metal more delicate and "soft", hence less deep inside the metal surface.So you should use progressive less coarser - or progressive finer stones - when sharpening. Two or three stone sizes should suffice for each kind of knives.
So, let's say that you need to have a total of 4-5 stones in your workshop plus a ceramic stone or a polish tape perhaps to finish off: a medium coarse, a fine-stone, an extra-fine, a "600" stone.
Then the stone grit depends on the kind of knife you want to sharpen . Here is what Ben suggests:
"Numbers" are country sensitive. The famous japanese honing stones (used for instance in the wonderful art of the Japanese "Samurai" swords - the "Takanas") are ranked by the "thousands". It's important to keep in mind the principle of "coarse", "medium", "fine", "extra fine" and so on, I think.
Here are the links to Ben and Chads website and tutorials:
Ben Dale's Edge Pro Sharpening Sytem
Chad Ward Tutorial
And to round off this page on how to sharpen a knife, here is what is universally recognized as best book on sharpening: