If it's low, it's leaking somewhere so get it checked out first. The best fluid is synthetic ATF(yes, tranny fluid.) Mobil 1, amsoil or Redline are good brands. Here's how to do a power steering flush and refill:
Jack up the front of the car so the wheels are off the ground and disconnect the power steering return line at the resevoir and place it in a bucket. You'll also need something to plug the fitting on the resevoir. Then start the car and let the system pump out. Turn the wheels side to side to get all the oil out. Now pour 1-2 quarts of regular ATF into the resevoir and let it pump out, turning the wheels lock to lock again. This will flush out the system. Now reconnect the return line and fill the system with synthetic ATF, run the car for a while, turn the wheels several times and top off the resevoir if necessary.