Lets look at the basics:
Lets start with Archimedian displacement theory.
Something floats if it displaces a mass of water with an equal or greater mass of water to itself.
Ice is less dense than water, thus the volume of the ice will be greater than the corresponding volume of water, and so it floats.
When the ice melts, the amount of water it displaced will be exactly the same as the amount of water that the ice has now become, so there is no net change in water level.
As for global warming, sea levels are supposed to rise because of the thermal expansion of the water; the ice at the north pole will have 0 effect, and the south pole will have little.