Best setting to get pictures in low lighting or florescence lighting?

I have a Cannon rebel XT and will be doing photography for a wedding, and have not done anything like this, i am trying to figure out the best setting to put my camera on for this kind of lighting

what i mean by i haven't done anything like this is, i haven't done a wedding all i have done is on location photography of children and familys in their natural settings

There should be no difference ... photography is photography For much of the wedding shoot you will be using your external flash (so you don't get "redeye").

Shooting in "available" light will of course cause you to change your white balance depending upon the light source and your ISO depending upon the amount of light.

I will assume you will be shooting RAW + JPEG to make your workflow faster, once you start editing (sorting) the photos

Make sure all your batteries are charged the night before and if your flash uses AA's bring spares with you

buy a sekonic 398A and KNOW your settings, dont guess.

a sekonic 398A is about $150.00 on ebay.

you can get a cheaper sekonic for about 40 or 50 dollars.

that is really your best bet you dont want to ruin someones wedding pictures.

The Rebel XT can be set for auto white balance. White balance is nothing you need to worry about. The auto setting will be fine and any color shift can be removed extremely easily in PhotoShop Elements which came with the Rebel I think. You should set your ISO on 1600 in the very low light and not bother with trying to use a flash at all. I am assuming you do not have an external flash because if you knew enough to have one of those you probably wouldn't ask anything here. The pop-up will cause red-eyes and the XT can do ISO 1600 pretty well. Any photo you hope to enlarge should use the lowest ISO possible. A spare battery and plenty of cards/film will be crucial. How "low" a lighting? A candle-light wedding will be beyond you. If an 80 year-old can see well enough to read in it you will be fine. Go to the venue and practice.

"Low light" could be anything from subdued natural light to low levels of incandescent (tungsten) light to candle light. All three require different white balance settings, so we can't answer. Taking a custom white balance should solve your problem, though. Then, I would suggest as close to a spot meter reading as you can get, so choose the 10% partial spot and zoom to the max to take your reading on the main subject. Lock the reading and then zoom back out or recompose and shoot.

If the reception is under fluorescent lighting and you will be using a flash, put a green "fluorescent" gel over the flash and set your white balance to fluorescent. This way, at least the people who are not in prime range for your flash will still come out the right color.

Good luck. This is a huge responsibility.