http://www.flickr.com/photos/little_pooky/3577674642/sizes/l/ Canon 5D

You can’t use flash inside–so a fast lens is needed.
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steve
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However, if you’re asking this question, then you are by no means ready to start shooting weddings. Keep up your research. Try to apprentice with a working professional. You’ll get an idea what is involved and begin working your way toward doing it yourself.
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What you should have to shoot a wedding: At least (2) camera bodies, primary and a back-up. You don’t mention the camera you are using, so there are some things you need to consider. You need to think about which cameras you will use, will they be full frame (or FX), or crop frame (DX), or both. The sensor crop will determine lens choice due to the "crop factor", or focal length multiplier. A crop sensor frame is smaller than full frame, requiring you to "multiply" the focal length by 1.5 to calculate "equivalent" focal length, or perspective. Ideally, you should have a fast aperture standard zoom lens, sometimes referred to as a "walk-around" lens. For full frame, a lens like the 24-70mm f/2.8 is a good choice. For crop frame, the 17-55mm f/2.8 is a good choice as it approximates the full frame equivalent focal range while it maintains the constant f/2.8 aperture. Next, you should always have a fast short telephoto prime lens like the 85mm f/1.4 for full frame, or the 50mm f/1.4 for crop frame as it approximates a similar perspective. Finally, you’ll want a fast tele-zoom lens like the 70-200mm f/2.8 VR, for use with either full or crop frame, one of the more indispensable lenses you could have in your repertoire. These lenses should be Nikkor lenses, if you shoot with nikon cameras. Next, you need light. I would recommend the Q-flash trio, from quantum, on a flash bracket. The trio is a hot shoe flash with more power, and a parabolic head and diffuser. It’s simply the best way to get good light at events.

That’s pretty much it, really, two cameras, three lenses, and a good on-camera flash with a bracket.

So put simply, you’d should have no more than 3 lenses: A wide zoom, 24mm (or equivalent) to short tele for moving around the event, capturing interest shots and candids. A fast prime, f/2 or faster, for low light and portraits, formals, creative shots. And a fast tele-zoom.

Important: Do NOT use slow lenses. You’ll notice I didn’t mention anything in the f/3.5-5.6 aperture range. The difference between an f/2.8 zoom, and an f/3.5-5.6 zoom is more often about $1000 or more, and the difference between shooting at ISO 200 (with little noise), and ISO 800 (more noise). Faster lenses mean cleaner images, in simple terms.

And I’ll tell you what a very good professional photographer friend told me. Don’t dilute the market with low cost, low quality work because you’re neither experienced, nor have the money to invest in professional equipment, just to occupy some spare time or fleeting interest in your life, all so that you can call yourself a "wedding" photographer. There is no "amateur" grade in wedding photography, no "beginner" fee to charge your client. If you’re not prepared for your client to pay you, at a minimum, $1,200 for a 4 hour event when you start, don’t even bother. Because there’s no way I’d pay you to shoot a wedding with a sigma lens because you can’t "afford" professional nikon glass. And that’s only because I know better. The problem with people like you is, your clients don’t. They don’t know that your not worth the time wasted. There are plenty of people [like you] who want to be professional photographers who can’t "afford" professional gear, but assume you should have the right to go out and do the work of a professional. We can spot you a mile away. These are the questions that if you have to ask, you need to STOP, go back to the drawing board, and research further on what you need to do a wedding photography event. Asking this question on yahoo answers is easy, because I just told you which lenses you should have. So technically, I answered your question. But thing is, most people who work as photographers never had to "ask". They took the time to learn photography FIRST. When you learn, you don’t have to ask………you’ll already know.

So please stop pretending you’re a photographer. If you have to ask what lenses you need, you’re at least 2 years away from trying your hand at shooting IMPORTANT moments in people’s lives for cut-rates because you have no skill. It really incites resentment in a lot of people who work hard at doing this.

In the end, you should do what you think is right. Or disregard this rant and do what you want. But if you don’t put forth the effort, you’ll never learn anything. So you know what lenses you should have, now what? What are you going to do with them? I suppose that’ll be your next question, huh?
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A wide-short telephoto fast zoom, like a 24-70 f/2.8, and a fast telephoto zoom, like a 70-200 f/2.8, are the basic kit. Preferably on two bodies, so you can switch between them as needed without having to stop and change lenses. Some like to have a fast 50mm or 85mm in their kit for low light work without flash. Some also keep an ultra wide angle (starting at 10-12mm) for a couple of shots.

Both bodies need a flash, preferably something powerful and diffused.

And of course, backups for all of it.
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edit:
I must say that mixedmoj is right (gear wise) and we should always get ourselves the best gear possible.
but the reality is that some people are looking for cheap wedding photographer (as some are charging $1200) and some photographer are looking to get some practice for weddings as they don’t have it. I would love to get my hands on some G glass or Zeiss one for the alpha 900 that I cant afford. but the choice of fancy gear, for some, is just gadget porn. I don’t see myself charging for $1200 to pay back my fancy gear that I paid on credit card to satisfy the "needs" of a "good" photographer and in the end screw up on something that I didn’t get the practice I was looking for because I wanted to charge $1200 and nobody thought I was worth the price.
I would love to get all of that, but I don’t $4000 to get me just started.
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portrait amateur, 50mm f1,8 user
http://www.flickr.com/photos/24095495@N07/
http://www.albin-roussel.com/

I prefer Nikkor but Sigma will work too. Of course, the proof is in the pudding. If the Sigma is capable of producing an image quality that your clients deserve then go for it. In my experience the Nikkor has better color accuracy and much better contrast than the Sigma. Also, in a professional situation the Nikkors tend to hold up better. Nothing worse than having equipment failure during an event.

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If you are serious about wedding photography then you may want to consider the following as a basic hardware set-up:

FF dSLR (and back up camera)
24-70mm f/2.8
70-200mm f/2.8
50mm f/1.4
85mm f/1.8 (or f/1.4)
High end OEM Flash

For much less than the cost of a new car you can be in business. Figure spending no more than $10,000 on hardware to get started.
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If that doesn’t work then get some couple friends and do some "mock" shoots, engagements, bridals, etc to help you gain some experience in posing, what’s flattering, etc.
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Second shooter for two different pros.