What kind of lens for photographing a wedding?

I have been asked to take wedding photos for the first time. One wedding will be in the summertime outdoors and the other in December and most likely inside. What kind of equipment works best for indoor events? I have a Nikon D90 with the kit lens and a prime portrait lens.
Also, since I have never taken wedding photos before, any additional advice would be appreciated!

The kit lens (presumably about 18mm to 55mm) will do most of the work, and the (presumably) 50mm f1.8 lens will be good for portraits, but you won't use it much. You really don't need much more in the way of lenses, though if you really want to spend money get a 17mm to 50mm at constant f2.8.It is unlikely that you will need a tele zoom, especially indoors or even outdoors on a cold December day.

Get a flash gun, don't rely on the flash in the camera. Possibly get a flash extension cable or wireless trigger to enable flash to be used off the camera. Get or make a flash diffuser or soft reflector. Get several sets of batteries for the flash

Get some practice in handling people if you can - that will be your chief challenge. Never upset the parents of the bride or of the groom! Preferably get to know the parents and other close friends and relatives before the weddings.

Check your camera battery capability, ideally have a spare, certainly take the charger with you. Check that your memory card has enough capacity for several hundred high-quality shots.

It's their wedding. You really shouldn't take the job if you aren't sure how to do it. Those are their precious memories. Tell the silly things to hire an experienced photographer, and you can take classes and learn to be a wedding photographer that way. :/

You can use a telephoto zoom lens! Why? Well, with a telephoto zoom lens you can capture almost everything at a much faster and crisp way. A wide angle lens will fit in too but the bad thing about them is that they result in a too wide or a fisheye effect on the photograph.

A telephoto lens will give you wide range from capturing the portraits to capturing the further happening things by the bride or groom's family members or something like that.

I would suggest a telephoto zoom lens.

Have a great day at the event!

It takes a lot more than just a certain lens:

http://www.christophermaxwell.com/wedding-photography-tips.htm

http://photo.tutsplus.com/tutorials/shooting/10-top-tips-for-the-amateur-wedding-photographer/

http://www.rokkorfiles.com/Wedding101-page1.html

If you have to ask this question, you are not ready to photograph a wedding!

Are you being paid for this? I. E. Are you the hired 'pro'?

You need far more gear & experience than you seem to have. My minimum gear for a wedding is;
- two DSLR bodies (one as a backup)
- fast primes or zoom lenses - 50mm f1.8, 85mm f1.8, 24-70mm f2.8 and 70-200mm f2.8
- at least 2 off camera speedlights plus modifiers
- plenty of spare camera & flash batteries, & memory cards

@JohnP - the kit lens will do most of the work? Really? Indoors? What if flash is not allowed during the ceremony? Sorry but the kit lens is just not fast enough for indoor shooting if you can't use flash.

Here is the minimum kit you should have.

- Two Mid to high end DSLRs
- 2 TTL Flashes
- 24-70 f2.8
- 70-200 f2.8
- 50 f1.4
- Junk lens (in Canon I recommend 24-105 f4, not sure about Nikon) as emergency backup
- spare battery for EACH camera
- plenty of batteries for the flashes
- tons of SMALL memory cards (4GB… Maybe 8GB at the most… Each)

That is the bare minimum kit you should have to shoot a wedding.

Do you have profesional liability insurance?
Do you have properly written contracts?
Do you have the skills necessary to capture quality images under less than ideal conditions at a moments notice?

You may be in over your head.

ps.: The Kit lens will NOT do most of the work.