Wow, there's a lot of not knowing there, so it's tough to tell you where to start. Did you buy it online or at a camera store? If you got it at a store then stop back with your best "I'm confused" face and let them know what you're trying to do and see if they'll take the time to give you a quick tutorial. There are likely plenty of "Getting to know your DSLR" video tutorials out there so you can search YouTube. I also suspect others here will have some bookmarked. There may even be similar threads elsewhere on this forum, so browse the D5100 and Learning Photograph forums to start.
Quick rules?
1. The higher the aperture number the more light required, and the deeper the depth of field. If you only want the subject in focus go with the lower numbers and you'll be able to capture the action without blurred subjects because of motion.
2. The higher the ISO number the less light required, so you can use faster shutter speeds to capture the action. The cost is that there is more digital noise the higher you go.
3. 1. For sports if you want to freeze the action then you need to use faster shutter speeds. Think 1/250 or 1/500 of a second or faster. Set your camera on 'S' for Shutter Priority (you set the shutter speed, the camera chooses proper aperture for you), set your shutter speed and check your meter to make sure it doesn't read LO.. Raise the ISO until you get the aperture reading you want. If it's dark in doors you may have to deal with ISO in the 1200+ range, which may give you a lot of noise. If you're close enough, use a flash to help out.
Read the manual. And I haven't tried downloading it, but Darrell Young's D7000 book helped me tremendously, and I found
this ebook he did on the D5100. Maybe give that a read?
Keep trying. You'll learn from your failures. Digital photos don't cost like film used to, so shoot lots and try not to get frustrated.
