Is the Nikon D5100 even a good camera?

My dad bought me this and some cheap lens that cost $400 that was recommended by some man that sells cameras.

What lens can I buy to take good pictures?

I know nothing about photography.

The Nikon D5100 should be a very good camera.
http://www.dcresource.com/reviews/nikon/d5100-review
Knowing nothing about photography makes it hard to fully appreciate all the adjustments it has available, but you could turn the knob to Auto and get good results most of the time.
I'm not sure what you mean by "cheap lens that cost $400". That might be the 18-105mm VR, which at $400 is not very expensive as lenses go, but not exactly cheap. It is often considered a good match with the D5100.
Whether you got a good sample, I don't know.

The D5100 is an excellent camera, no question.

Since you did not tell us what lens(es) you have we can't comment on if the lens you have is "any good", but usually a $400 lens is better than the standard lens kit.

"What lens can I buy to take good pictures?"

It does NOT matter what lens or camera you have if you want to be able to shoot "good pictures". Shooting brilliant photos has all to do with your skills using the camera, NOT the camera you use

Nice of your dad to buy you a gift.

You need a Nikon D4 Camera and a Nikon AF-S 24-70mm f/2.8 Lens.

I'm sure nothing cheaper will satisfy your skill level.

Since you admit to knowing nothing about photography it's pretty obvious why you can't appreciate an excellent piece of equipment.

If you can't take good photos with this camera an lens you really have a problem. I know professionals who use a setup like yours and are published.

I'd suggest you get some formal training before investing any more money - that's obviously what you need. But then, a professional who knows how to actually take photographs can make very nice photos with a $100 dollar pocket toy. But someone who knows nothing about their gear, hasn't taken the time to learn or isn't willing to learn can't make good photos with a $35, 000 camera.

Get some training.

Consider yourself very lucky to have what you do! The D5100 is a very decent entry level camera.
You can do a lot with the lens that came with it.
Try going to the library and see what kind of books they have for beginners.

You deserve better than such a cheap camera. Ask dad to buy you a Hasselblad HD4-200MS with a 80mm lens.

If you know nothing about photography then even a $10, 000 lens will not give you good photos. You need to learn about photography and how to use your nice camera.

Its one of the newest Nikon's released, to help you learn more about your camera and how to get the most from your dad's investment, get him to buy you this DVD it will take you beyond the words of the manual and teach you to take great pictures
http://www.quickprocameraguides.com/p.Nikon.D5100.html