Nikon D5000 clear lens cover?

I'm totally new to this camera and photography. I just bought a Nikon D5000 and am slowly leaning how to use it. I have been taking outside portraits of people with a 18-55mm lens. Is it better to use between 50-55mm for portraits? Will the pictures come out better?

Also, someone told me that there's a clear protecting cover you can shoot with that screws onto the end of your lens. Where can I find one? And do they effect the quality of photos?

Anytime you get a lens, always get a glass UV filter for the front of the lens. It will protect your lens from scratches and enhance the quality of your photos.

On the portraits, the closer you can get, the better. When I do portraits, I use a fixed 85MM lens. The image quality for a fixed lens is a much sharper and 85MM allows you to get very close while standing at an acceptable distance.

UV filters are a waste of money. Any impact strong enough to damage the thick front element of your lens will easily smash through the very thin glass of a UV filter. In addition, unless you pay $50 or so for a high quality UV filter, the inferior coating on a cheap filter can introduce flare to your photos. Get a lens hood for your lens if you don't have one and always use it.

Yes, that's a good focal length for portraits. You shouldn't be content with just being told that it's a good focal length for portraits, but also want to know why that is so: It's because from approximately this focal length on (as well as longer ones) there won't be a significant distortion through perspective.
Think of the opposite, taking portraits at 18mm. Then you'd have to get very close to the face, resulting in a "big nose" effect. At the long end of that lens this effect doesn't happen.
As an additional benefit, using long focal lengths contributes to blurring the background.

That protecting cover would be a UV filter. It is screwed on the lens so that the actual lens front element stays clean of dirt. If the filter gets dirty and you scratch it while cleaning it, or if something hits it and scratches it, then it costs $30 to replace the filter.It'd cost hundreds of dollars if the same had happened to a good quality lens.

With filters, only buy quality brands. Hoya and B+W are reputable, they won't noticeably affect the image quality. Take care that you buy the right filter thread size. Your 18-55mm kit lens uses 52mm filters (this is also written on the lens itself)

Edit:
IMHO the actual protective effect isn't supposed to protect from impacts strong enough to break, but from things that can cause scratches. E. G.a passing car making a small rock fly towards you, or tough dirt stains that have to be rubbed off.