In the absence of a diagram the general answer to the roller coaster scenario is as follows. Energy is always conserved in nature and if we exclude the energy losses due to friction in the moving parts and surfaces of the roller coaster, the sum of kinetic and potential energy of the car will always equate to the same number, called the mechanical energy.
As the rollercoaster moves you can imagine energy flowing between kinetic and potential states. The highest potential energy will be the highest and slowest point on the track, this will also be the lowest kinetic energy state. Similarly, the highest kinetic energy will be the lowest and fastest point on the course, which is also the point of lowest potential energy.
