First make sure you have enough power steering fluid. If so, then with power steering fluid cap off, turn the steering wheel back and forth to full lock around 10 times to purge any air that may be trapped in your power steering system.
If that doesn't help, then your power steering fluid (automatic transmission fluid) needs changing. It has turned to sludge. Sometimes this happens when people put power steering fluid in their power steering reservoir instead of ATF fluid.
In either case, if you look in your reservoir I'll bet it's brown and muddy looking.
The problem is, there is no actualy way to drain your power steering system without disconnecting a hose. Furthermore, the power steering fluid filter is a part of the reservoir itself (the bottom half), so you'll have to change the whole reservoir (around $30 from the dealer) to change the filter.
The good fix:
buy a turkey baster from the dollar store
buy 1L of ATF fluid (around $8 from Canadian Tire).
Suck out the old fluid up to the filter with the engine running.. you'll see fluid moving in the reservoir as the vehicle is constantly pumping fluid. Top it up again, then turn the steering wheel back and forth around 5 times to full lock to purge the new fluid in the system (and mix with the old). Suck out the half-clean mix, top it up, and keep repeating. Do this around 3-4 times, until the fluid begins looking red/purple again.
The better fix:
disconnect the power steering hose and let the car pump out all the fluid in the system. Top up the reservoir again, and start the car... the fluid will be sucked into the power steering lines and pump, keep topping it up. Now with the cap off, turn the steering wheel back and forth 10 times to lock.. this will get the air out of the system. Top up the reservoir again.
The best fix:
Do the above, plus change the power steering reservoir so you'll have a clean filter, and won't have old mud deposits still in your system.