I've never photographed jewelry but I have figured out a system for other items (I sell small items such as cat collars and belts... you could use a similar setup.)
You want light - but not direct light. This would cause harsh shadows and you don't want that. You want soft, diffused light. You could buy a fancy "photo tent" to diffuse the light... but I personally didn't want to spend that much money. What I use is a semi clear rubbermaid container (you could use a smaller size - but I use mine for purses as well). The plastic is sort of frosted - and this serves to diffuse the light somewhat. I also drape a layer of white interfacing over the container (you can get this at any fabric store - about $1.50 a yard. You could also use a white sheet. Onto the container I clip two lamps with daylight light bulbs. They produce a much nicer light than normal incandescent (which produce a yellow cast) or fluorescent (which produce a green cast) light bulbs.
For the background a cut down a piece of white posterboard to fit the container - and curved it inside. This is so there will be no seam between the bottom and back sides of the container. It will look like a seamless background.
If you can, use a tripod. If your light is not bright enough to shoot faster than 1/30s shutter speed - then you need a tripod otherwise your photos will be blurry because not many people can hold their cameras still enough for slow shutter speeds.
My suggestions - know your camera and how it works. For jewelry, you will definately want to use the macro mode (usually a tulip icon). If you have the option of manual settings, learn them and use them - as sometimes automatic mode won't produce the best piece. The better your initial photograph is, the easier time you will have later on. Also have and learn photoshop. If you don't want to shell out the dough, get Photoshop Elements. You get pretty much all the same features for a fraction of the price. You want to know the CURVES feature inside and out - this one (as opposed to brightness/contrast) will give you the most flexibility in working with your photos.
Like with any craft, photography takes practice. If you just keep trying and experimenting with what looks best, you will notice your photos get better and better. The photos I first used on my site looked good then, but if I looked at them now compared to my current photos, they look horrible.
Hope this was helpful and not too confusing! Good luck!
Here is a photo of my photo setup.. just for reference.