What you see is what is called "noise", and is a resultant of having a high ISO on a camera that does not have sophisticated noise reduction on high iso levels. Obviously the camera is in an automatic mode, most likely with the camera automatically setting the ISO level. Thus, you need to learn how to use manual mode, it will take some time, but it will greatly help you with future photography. Basically to you always want to have as low ISO level as possible without it seeming noisy. On a camera such as yours, you really don't want to be going any higher than 400 on the most. To be able to put your camera on something such as 100, 200, or 400 and still have a fast shutter speed you need to be have you aperature as open as possible...here is where i will break to explain the use of the aperature.
Think of the aperature as simply this: your lens has a hole to let light go through it. This hole can be expanded and contricted. By expanding the hole (or what is referred to as the aperature) you are letting more light into the camera to be recorded. Play around with the aperature going up and down in the numbers and you will begin to understand how it influences the shutter speed.
Note: you need to also understand the a higher number on your camera means a smaller hole, i know seems the opposite way it should be, but just remember the smaller the hole, the larger the number. Thus, the bigger the hole the smaller the number. I will give you a few examples:
ex1. when your camera reads f22, the hole will be fairly small, thus allowing very little light through the aperature, and thus making you have a slower shutter speed which is usually not ideal.
ex2. when your camera reads f5, the hole is pretty much all the way open on your lens, making it able to attain a higher shutter speed with proper exposure.
Now you might be saying how do i know if i need a wider exposure and slower/faster shutter speed. Well, while in manual mode while looking through the camera there will be a meter on the bottom of the screen which tells you what your exposure will look like. I am not sure about nikon but you will figure out which scrolling through your shutter speeds if the higher the shutter speed the line moves to the left or the right. The goal of changing your shutter speed and aperature is to get that line right in the middle, thus making a correct exposure (Note: not always is this the correct exposure, but 99% it is near correct)
So now you have had a chance to see what effects aperature and shutter speed have on each other and how the ISO number impacts these two factors.
HOPEFULLY this long essay i just wrote out helps...if not well ask a more specific question or feel free to PM me.
