Possible causes for a stiff steering wheel are a loose or damaged power steering belt, low power steering fluid levels, a damaged or leaking power steering rack or a broken power steering pump. Some of these issues are more prominent when the vehicle is slowing down; if this is the case, it should be fixed as soon as possible.
If steering is an issue when slowing down, check the power steering belt. If it has slipped or is loose, it can cause heavy steering, but if it is loose, it is likely to cause a loud screech, especially when the car has just been started when it is cold. If the belt is loose, it will likely have worn down and should be replaced; however, it can be adjusted to stop it from slipping. If the belt is damaged, it will need to be replaced in a short amount of time. Replacing it as soon as possible may save time in the future.
It is a good idea to check the power steering fluid levels at regular intervals and refill it if necessary. If the fluid is low, it can cause harder steering. If the fluid gets low soon after it has been refilled, there could be damage to the steering rack and the fluid is leaking out. Check the steering rack, and if there is any damage, replace it right away. The power steering pump should be checked to see if it is keeping the right amount of pressure. If not, it needs to be replaced.