How to focus on part of a picture?

This is an example.
http://www.wisemanhousechocolates.com/february-valentines.html

Like in this picture, I also want to take pictures of chocolates, but I only want one of the chocolates or just the center to be in focus. I have gotten close and used the highest aperture or the one with the lowest number ie.f4. I have tried a lot of different things but to no avail. I'm not sure what else to do. I would like how the front of the plate and the other chocolates are a bit out of focus. I have a Nikon D3000 with a 18-55mm lens.

Put it on macro, let it focus and quickly snap the picture.

Your lens is not a macro lens, so close ups like that one will be difficult.

You can use some diopters on the front of your lens that will allow you to get closer to your subject.

What you are seeing is called selective focus and is usually done using lens apertures that are from wide open to one or two stops down.