As already mentioned, which whetstone you need depends on the sharpness of the knife. Is your knife still in good condition and you just want to sharpen it a bit to make it sharper? Then you only need a whetstone with a fine grain size. Is your knife really dull or damaged? Then you first need a coarser stone to get your knife back into shape. mouldback in. Then, in increasing grain size, you can make sure your knife is razor-sharp again!

To see what grain size you need, you can easily test this with a tomato. Because a tomato has a tough skin, but with a soft interior, this vegetable is how good indication of the sharpness of your knife. Slice the tomato without putting pressure on it and see how easy (or difficult?) it goes. 

• Grain size 400 - 1000 (coarse to medium).
• Grain size 1000 - 3000 (medium to fine) 
• Grit size 3000 - 5000 (extra fine)  

Then, to keep your knives in good condition for as long as possible, I recommend you maintain your knives daily with a sharpening steel.

Sharpening angle
If you sharpen the knife with the wrong angle, you can damage the knife. There is also the danger that the knife can become blunt if you sharpen the knife with too large or too small a cutting angle. You determine the right sharpening angle by looking at the angles of your knife. After all, the smaller the angle, the sharper the knife. For example, the angle of a Japanese knife is very small because of the hard steel, and so the knife can be sharpened very sharply. European knives are made of a softer type of steel and are therefore sharpened at a larger angle. 

• Double-sided sharpened knife: sharpening angle of 20 - 25°
• Single-edged knife: sharpening angle of 15 - 17°.