I'm not sure about what lens I should get?

I'm having a Nikon D90 with a Nikkor 18-55mm lens.
And I'd like to know if the Nikkor 55 - 200mm lens would be an option to the Nikkor 18 - 200mm one, or if I should just go for the 18 - 200mm?

The 18-200mm is going to be sharper but more expensive. You already have the 18-55mm range though, so you would render your 18-55mm useless if you got the 18-200mm.

I recommend the 18-200, then sell your 18-55mm.

OR you could go for a different telephoto option altogether, like the Tamron 70-300mm.

You should also consider the aperture of the lenses you are looking at when shopping for your second lens. If you have had problems with your current lens not performing well in low light situations you might consider buying a fast fixed focal length lens like a 28mm f/2.8 or 50mm f/1.8. Fixed focal length lenses are usually faster and almost always sharper than than zoom lenses. If you go back and you seem to be having trouble in low light more than with getting close enough, I would suggest buying a faster fixed focal length lens.

If you go back and look over the images that you have taken with the lens that you have now, can you say that you are mostly happy with the range of the zoom you have now or are you constantly not able to get close enough? If you need the extra range more often than not and you haven't run into any issues with shooting in low light then you could consider buying the 55-200mm or 18-200mm but I would not personally recommend either of these two. They just aren't fast or sharp enough for my tastes. For that price you could most likely find a decent used 80-200mm f/2.8 which you will be much happier with.