From your question I'm going to guess that you're using "P Mode" and allowing the camera to make the decisions about aperture and shutter speed. Since Nikon wants your pictures to be correctly exposed and free of blur caused by camera shake, "P Mode" is biased towards a faster shutter speed.
If control of your camera is what you want then you need to try one or all of these modes: "A", "S" or "M".
"A" is Aperture Priority. You select an aperture and the camera selects a shutter speed that will give a correct exposure based on the ISO used. This mode is best used when you want to control depth of field (landscapes, groups of people) and shutter speed isn't as important.
"S" is Shutter Priority. You select a shutter speed and the camera selects an aperture that will give a correct exposure based on the ISO used. This mode is best used when you're shooting action and want to control how much the subject is or isn't blurred from its motion.
"M" is Manual Mode. You check the camera's light meter and manually select an aperture and shutter speed that will give a correct exposure based on the ISO used. This mode allows you full control of the exposure.
Read: http://www.picturecorrect.com/tips/aperture-priority-shutter-and-manual-mode/
You've probably noticed use of ISO along with aperture and shutter speed. These three terms make up the Exposure Triangle. Learning it will definitely help your photography.
Read: http://digital-photography-school.com/learning-exposure-in-digital-photography
The idea is that you need to take control of your camera and try using "A", "S" and "M" modes and forget "P" mode exists.
This old rule is also useful to know: "Sunny 16 Rule". It was invented when cameras didn't have light meters and few amateur photographers owned a hand-held light meter. It states: "On a sunny day, set your aperture to f16 and your shutter speed to 1/ISO."
Using ISO 100:
f32 @ 1/25 sec.
f22 @ 1/50 sec.
f16 @ 1/100 sec. "Sunny 16"
f11 @ 1/200 sec.
f8.0 @ 1/400 sec.
f5.6 @ 1/800 sec.
f4.0 @ 1/1600 sec.
Understanding the Exposure Triangle will allow you to know why the shutter speed gets slower at f22 and f32 and faster at f11, f8, f5.6, etc.
If you'd like to experiment with motion blur to give a more dynamic look to action pictures read the following:
http://digital-photography-school.com/a-beginners-to-capturing-motion-in-your-photography
http://digital-photography-school.com/how-to-capture-motion-blur-in-photography
http://digital-photography-school.com/mastering-panning-to-photograph-moving-subjects This technique takes some practice but the results are worth the effort.
Watch this video tutorial: http://www.nikondigitutor.com/eng/d3100/index.html Who better than Nikon to teach you all about your Nikon D3100?