Aperture help needed

I use Nikon D80. Lens: 18-135mm so what is the best aperture for potrait? Kthanks

For your lens, the lower the aperture, the better. If you can set up to where your setting is at f3.5, that is probably best for your depth of field. Adjust everything else accordingly; aperture is likely your most important setting for a portrait.

You should also be concerned about focal length, not just aperture. Anything above 60MM will be fine for portraits.

The smaller the f-number, the better. If you're using the kit lens, chances are you probably can't go much lower than f/3.5. This will still give you a nice portrait and will be noticeable, but if you can, try to get around f/2 to f/1.8. That will be the most dramatic Bokeh effect, and your subject will be nice and sharp, and the background will be nice and blurry. You can always go higher if you don't like it.

There's no such thing as best aperture, even when you're going to blur backgrounds. Exposure values aren't constant. They depend on multiple factors like light amount and quality and subject matter. At the moment, go back to Auto mode. Venture into aperture priority when you have read this.

http://www.illustratedphotography.com/basic-photography

Aperture is part of the trilogy of exposure settings called the "exposure triangle". Aperture works in conjunction with shutter speed and ISO to provide the correct exposure.

Within that framework, the aperture will allow you to control the depth-of-field. Sometimes you want a shallow DoF, and sometimes it is too shallow. So as aperture is a dynamic setting, there's no one best aperture.