A circular polarizer can be useful, if you want to cut down on reflections on glass, water and plant life. In addition it will bring out deeper blues in the sky on a bright day (as long as you are at right angles to the sun). It can also be used to slow down the shutter speed, but not by much, maybe -0.7ev to -1ev.
From the specs of the 18-35mm Sigma you need a 72mm sized filter for that one. Being a somewhat wide angle lens, make sure you go for a wide band filter, since they are thinner and less likely to cause dark corners in shots at 18mm. I'd recommend getting a multi-coated Kenko, or Hoya filter, (Keno is the parent brand of Hoya). They are the same product, but the Kenko is often sold for less because Hoya is more well known outside of Japan.