Taking pictures in sunlight with Nikon D3100?

I just got a Nikon D3100, and I was wondering what are the best settings for taking pictures outside when the sun is shinning? Mostly pictures of people, because my sisters prom is coming up, and I want to know in advance so I know what I'm doing when the time comes.

Use the sunny 16 rule

f/16 where shutter speed is about the same as the ISO. If you are doing more portrait like shots, use a larger aperture and faster shutter speed to create a shallower Depth of Field.

there's no one best setting, you have the change ISO, shutter speed, and aperture to conform to the situation

It is very very dependant on the situation. Yes there's the sunny 16 rule but I question if with modern cameras that is still needed. Still F16 can be very useful to get as much detail as possible [of course for portraits you want it wider to let the background turn soft]

Even the use of ISO is well if you for instance use flash the ISO setting effects how far the flash reaches.

You know keep it in Program Mode or Automatic mode for your sisters Prom. Maybe Aperture if you want to at least control the aperture. But trust your camera to decide what probably is the best setting for a specific shot. That way you can make pictures, with and without flash and get decent results.

If you take control you must know what you are doing and sometimes you just want to have fun. And put things on automatic.meaning less worries for you. Sure automatic doesn't always get the best pictures but in many cases it actually makes very decent choices. And you have to remember you are there to have fun to right?

So what can I say? Trust the camera it will give you very decent results.

Try experimenting in aperture priority(Av) mode and setting your ISO to somewhere between 100 and 800 to start out, or if you're uncomfortable with that go with auto ISO('film speed'). Aperture priority mode means the camera will determine which shutter speed you need relative to which aperture setting you've chosen. The smaller the aperture the longer the exposure will be. It also increases the depth of field as you stop down. This gives you the ability to choose how isolated your subject will be from the rest of the scene without having to worry about a full manual exposure. There's a lot more to taking a good photograph in digital than that, so if you've got the time go to the library/bookstore and get a photography book, it will help a ton. Learn to make your own exposures and you'll give much, much better results than from auto/easy mode, good luck!

Turn the ISO down to its base ISO of 100. Besides minimizing noise you will get best dynamic range, which can be important in the contrasty sunlight. I don't think the 18-55mm comes bundled with a lens hood. That is sold separately. For situations where the sun is in front of you, a lens hood can be helpful in preventing lens flare.
With my D90 I find that in the Shooting Menu, setting Picture control to Portrait gives me best flesh tones. Turning down the contrast in the picture control menu can improve the dynamic range in the bright sunlight. The pictures can look a little bit dull out of the camera. Tinkering with levels in software can make them more snappy. It's easier to bring up the contrast in software if needed than to try to retroactively soften it. You could also experiment with the Active D-Lighting settings. Sometimes they help, sometimes not.

Practice taking pictures before the prom arrives.