I have the canon EOS 30D SLR camera. would like to do weddings. What lens should I buy.?

Please go to this page and read an extensive discussion about selection of a lens or lenses for weddign photography. You will find it very helpful. If you can't afford the "L" series lenses yet, at least use the focal lengths mentioned as a guide.

http://www.the-digital-picture.com/Canon...

It's all about the lighting, though, as fhotoace will surely tell you when he answers this question.

EF 28-135mm 3.5-5.6 IS USM $400...........EF-S 17-85mm f4-5.6 IS USM $515..........

I would suggest a fast, wide angle (non distorting). Look for something with f/2.8 or less over the whole range. Since weddings are often indoor and low light, the faster the lens the better. Of course, you have depth of field issues as well, but unless you've got a massive flash and don't mind temporarily blinding the wedding party, you're going to need a fast lens. I have a Sigma 18-50mm f/2.8 for my Rebel XT and I love it. I think it only cost around $400 at B&H.