Nikon d5100 help! New user and confused?

For some reason my camera will not focus.help?
i'm new to pro.cameras so i'm clueless.also, can someone explain the difference between the M A S and P settings on the camera? I've read about it but i'm still confused.thanks

Added (1). I know this will make me sound really stupid but i don't care. The main reason why i don't understand when i read about it is because i don't get what ISO, aperture, and shutter speed is/does.

Regarding focus:
1. Check if your lens is set to Auto focus. On the lens itself you should see a tab that reads AM on the left side
2. Check if your camera itself is in autofocus mode. Check your manual over how to do so

M is complete manual, where you control aperture, shutter speed, and ISO, upon other things
A is aperture control- where you choose the aperture, and the camera chooses the shutter speed
S is shutter speed control- where you choose the shutter speed and the camera chooses the aperture
P is program auto- basically auto but you can control when you flash, when you don't, if you want to over expose, etc.

I use my camera on M

no problem. Ill give you a short, one sentence rundown with a couple of examples
Shutter Speed is the amount of time that light hits the sensor. The faster the shutter speed, the more light you need, but you will get less blur. 1/2000th of a second is enough to make helicopter blades at full throttle stand still in the picture.

Aperture is the amount of light allowed through the lens to the sensor. The larger the aperture, the faster the shutter speed can be, since more light is coming through. Likewise, the smaller the aperture, the slower the shutter speed

ISO is the camera's sensitivity to light. The higher the ISO, the less light you need, but the noisier the picture. The lower the ISO, the more light you need, and the cleaner the picture.

Lets say that you are taking a picture, and you use a 1/200th of a second shutter speed, f/8 aperture, and an ISO of 400 to get a proper exposure.
if you make the aperture larger, you can lower the ISO to get a cleaner picture OR you can shorten your shutter speed, or both if you are allowed to.

if you make the shutter speed shorter, you will have to raise your ISO or make your aperture larger

if you raise your ISO, you can make your aperture smaller, or shutter speed shorter

likewise, the opposite will happen as well
if you make your aperture smaller, you will have to raise ISO or lengthen the shutter speed
if you make your shutter speed longer, you can lower your ISO or make your aperture larger
if you lower your ISO, you will need to make your shutter speed longer or make your aperture larger

I suggest a book called "understanding exposure"