As mentioned, check your fluid level.
If it's ok, get the power steering system bled - this is best done at a mechanic. From here one of three things will happen -
a) it fixes it, hurray!
b) it fixes it for a while, then it comes back. You have a leak in the system somewhere, mechanic can advise. Can be expensive to fix, cheap & temporary fix is to put an additive to your PS fluid to clog the leak.
c) it doesn't fix it at all, then your power steering pump could be shagged.
Cheers.