if it's the same 28-135 that I have (don't think they make a different one) you'll need a 72mm filter. But as big mike said, look at the front of the lens, it will be printed around the outer edge
Also do yourself a favor and don't buy the cheapest filter you can find... at least get something multi coated (like the Hoya HMC line). Do a forum search for more info, there are some good discussions on the subject.