How to create a high resolution 300dpi image?

I'm using a Nikon D3000 DSLR Camera

Use the highest resolution when you take the photo, and then later in an image editing program such as photoshop open the image and save it as a 300dpi one.

DPI is just an instruction for the printer about the scale. It basically tells the printer to scale the picture so that 300 pixels along each axis are supposed to be one inch long then. This does not say anything about the actual size or pixel dimensions of the photo.

Typically, any lab that asks for prints of a certain DPI figure to print the photos for you should be able to set them to that value themselves.

Even a cell phone can create a 300 dpi image. Any digital camera can.

I think you should read up on the definition of DPI.