How to get nikon d5100 to focus on subject?

Every time i try to take photos of flowers my nikon won't focus on the flower. It focuses on the backround instead.
i even try putting it on different modes (the scenes and effects)
i also tried changing the lens to manual mode instead of auto but nothing works!

anyone know what i could do to get it to focus?

Try extension tubes or a macro (Nikon call them Micro) lens.

You can't focus because you're too close.

If you're trying to do macro with your kit lens (I'm assuming it's a kit lens) it won't focus because the lens doesn't focus that close.
You would need an extension tube, macro filter or macro lens. You could also reverse the lens, but that's a little harder.

Move back.

You are too close. Don't zoom your lens out, your lens will focus closer when it's at the widest setting. Try this: put your focusing on Manual, set the lens focus to the closest setting, move the camera closer/further to your flower until it comes into focus.