You can do either. If you are a complete novice then just replacing the fan would be simpler.
To replace the heatsink and fan is not that differcult but can be a bit of a pain as you do have to remove all the old thermal paste. A new fan and heatsink unit normally comes with new paste already on it and its just a matter of removing a plastic protection on the heatsink and putting it on the processor making certain its on properly and level.
A fan can be got easily enough at computer fairs and local PC shops. If you are going to replace the heatsink and fan then theres a bewildering choice and some are virtually silent.