What three things must a planet have to be a planet?

To qualify as a planet, a body has to be approximately spherical (achieving hydrostatic equilibrium under its own gravity), it has to orbit the sun and it has to have cleared its orbit of all other objects - so that at that distance from the sun, there are no other sizable bodies. Some dwarf planets, like pluto, fulfill the first two requirements, but not the last one, this is why they are deemed dwarf planets rather than planets.