JUST CHANGED INTO A 50mm LENS. IT SAYS F- AND KEEPS BLINKING

I use nikon d90 and just changed it into the 50mm lens i just bought.

now the aperture value says "F - -" and it keeps blinking
it won't let me take any photos and won't auto focus

how do i fix it

That's the "No lens attached" error code. Be sure that you have rotated the lens fully until it clicks into place. If you try to rotate it without depressing the lens release button, a properly mounted lens should not turn at all.

The answer above has described the fEE error code. See:

You need to wind your *lens* aperture to it's smallest setting (F22 probably) and lock it over.

Edit: Yeah now I think of it, Sam's quite right, most of the newer 50mm lenses probably don't even have manual aperture rings. It's been that long since that error message has happened to me!
Discount the above, it concerns a slightly different problem with older lenses.

Gracious sakes, Allison! Changing into a 50mm lens must have been really traumatic for you. I'm glad you changed back so you could ask this question.

Try READING the Instruction Manual for you new lens and your D90. I'll bet a dollar to a donut hole the Manuals will tell you how to use your new lens.