The more sugar suspended in the liquid solution the higher the hydrometer will float. As the sugars are converted into alcohol the hydrometer will float lower and lower in the liquid. What you do is read the number on the scale at the water line. Pure water will read 1.000, pure alcohol will read about 0.800 so as the alcohol content rises in your mead your hydrometer is no longer measuring dissolved sugars precisely but it will still give you an idea of what's going on in your brew.
I wouldn't worry too much about gravity readings now anyway, you'll want to let it sit a few months even after fermentation has stopped to let all the yeast and other particles settle to the bottom. If you really want to know just follow what Halbrust said.
If you want a sweet mead you'll have to stabilize the mead by adding sulfate and sorbate, this kills off the remaining yeast. Then add small amounts of honey to the mix, tasting it after each addition until you find the sweetness you're looking for. 1.000 is dry, 1.010 is semi-sweet and 1.020 is quite sweet, add slowly to find the level you like.