Nikon D90 - how do I get pictures without it being blurry in the background?

I enjoy taking pictures of photos, whether it be nature, animals… I take a lot of pictures of turtles and birds, actually. My problem is this. I just got a Nikon D90 and the background on all my pictures is blurry. That is fine for some pictures, as it is clear on the image I'm focused on. But the problem is this. Sometimes there's one animals, with another animal in front of it or in back of it. The one I focus on is clear, but the other one is very fuzzy. Suppose I want the entire picture to be clear with no fuzziness? I have a 75-300 mm lens. What setting should I be on? What can I do to get an 100% clear picture?

That's actually a good thing. BOKEH! Anyway, close the aperture (i.e., increase the f-stop number). A narrower aperture will give you a larger depth of field. However, if you zoom in on something close enough (commonly happens with a telephoto lens which is what the 75-300mm is) and the background is at a significant distance front the object of focus, you will get a blurry background (BOKEH!) regardless of what aperture you use. Read this:

http://www.cambridgeincolour.com/...-field.htm

The 75-300mm is reputed to be not the greatest telephoto zoom lens, but the issues would be the same with any lens of similar focal length.
Depth of field is shallow at wide apertures (low f number) and long focal lengths. You can gain more depth by choosing a higher f number. There's a price to pay for this. You have to compensate with a slower shutter speed, which can show camera shake and motion blur, or high ISO, which adds noise. Long focal lengths tend to magnify camera shake, so hand held telephoto shots need good shutter speed to hide it. If the animals are resting a tripod can help. With higher apertures like f16 and smaller, you start getting loss of sharpness to diffraction.

Although the look youre achieving is often sought after by photographers, the way to undo would be to stay at 75mm (all the way zoomed out), stay far away from your subject, and keep the aperture closed down (around f11 or higher).

Depth of field - this is the area of the image that is in focus.

One important factor is the lens aperture (but not the only one).

The smaller the aperture, that is the larger the f number the greater the area of the image that appears in sharp focus.

http://aviewfinderdarkly.com.au/2011/04/27/how-to-use-depth-of-field-for-landscape-and-portrait-photography/