Hi, the size of the filter is determined by the diameter of the lens itself, not the lens' focal length (18-55mm). So look at the front of your lens. Most likely there will be a size printed on its rim where the glass meets the threaded edge.

Also, the range of filter quality does vary within each manufacturer. Look for glass, multi-coated filters (so as to reduce flair). A UV filter will protect your lens' glass from dirt and abrasion should you happen to brush against anything as well as cut down on haze. A polarizer will reduce reflected glare when used properly.