Trying to decide what kind of sharpening tool to get

Hello guys!

I've been looking to learn how to sharpen a knife, and with a little investigation, i've devided to go for a stone with 400 grain, and another one with 1000. The thing is, there's some stuff i don't understand.

Is really needed a diamond sharpener? What's the difference with that and a normal stone one? If i end up using one, any tips?