Welcome to the world of DSLR's, there's no turning back now!
The D5000 is an entry level, but very capable DSLR. You'll likely be very happy.
You said "extra lens" 55-200mm, so i'm assuming the other lens is the kit 18-55mm lens? Both of these lenses are very good at their respective price points, but they do have there limitations for certain types of photography. You're friend was certainly not wrong to recommend something with a "low F stop", otherwise known as aperture. The low F stop means the lens can gather a lot of light, so it's great for shooting in dim conditions (closer to dusk/dawn, indoors at night time under weak lighting, sports action shots, and they are also very commonly used to "blur" the background on portrait shots, a very pleasing affect that many are after, known in the photography world as "Bokeh") However many of the lenses with larger apertures (large aperture = low F stop) either cost significantly more and are very large and bulky, if they dont fit those two criterions, then it's very likely a prime lens, a lens that is fixed at a certain focal distance, so you can't zoom in and out with it...
Primes are a great option, because they can be very cheap, have good large apertures, and are very sharp... however they do come with the drawback of not being able to zoom... the nikkor AF 50mm F1.8 that you mentioned is undoubtedly a great lens, and a great value, HOWEVER, it will not auto focus on your D5000, it will me manual focus only, which might not be so great for a beginner. Nikon has a great 35mm F1.8 AF-S lens, that WILL auto focus on your body, at a very similar price point, I think you'd be happy with that lens. Nikon also came out with a new 50mm 1.8 AF-S lens that will also autofocus on your body, but it's a very new lens and i believe the price point is a little bit higher.
For future reference the only lenses that will autofocus on your D5000 are "AF-S or AF-I" lenses, like the two kit lenses you have. All of nikons newest lenses are AF-S, but some of the older better valued lenses are not.
I suggest you play around with the camera some, and get a feel for the limitations on the lenses and camera, and report back to us with what you found so that we can better recommend possible equipment or lens upgrades, the two lenses you have taking you from 18-200mm is a very effective range to have at your disposal, so it may be all you ever need.
Also, If you're not familiar already, take a delve into the world of DSLR's and obtain a good understanding of Aperture, Shutter Speed, ISO, Exposures, etc... the basics of photography outside of the "Auto" mode if you will...
Good luck, and let us know what you find!