Lets say I sharpend my knife starting with Grit 400, then 1000 up to 3000 and 6000 but its still is too dull.
If i want to sharpen the blade again do I restart with grit 400 or 1000 ? And is there a way to tell where to start again?
Grit means very little for sharpness beyond ~2000. Far more important is a consistent angle.
This is a good article that explains the angles
a good sharpening tool will have a angle guide, something like this
I'd recommend against "eyeballing" it the first few times.
Joerg Sprave made a nice video explaining a lot, he uses the Gatco set I linked.
And don't forget to hone your knife before trying the edge! In my experience that is the bit that pushes a edge from "eh, not bad" to "whoooo/
There is no set way of knowing exactly what grit to use but you can estimate. <1000 grit is for repair, so I would suggest visual inspecting the blades edge under a light. If there are any points that reflect light differently, or that you can see any damage, then use the 400 grit until the edge is consistent. If your knife passes the visual check, but struggles to cut consistently along the length of the blade (newspaper will be a good test of this consistency), then i would start at 1000. If your blade is already sharp enough to shave hairs but you want it to split atoms, start at 3000 but you may need to look beyond stones to leather strops and polishing compounds.