thanks for the quick response, can you recommend what lens i should buy?
Frankly, it depends on what you're shooting (portraits, candids or landscape), the light condition (day or night, indoor or outdoor) and how near you can get to shoot from.
If you want to shooting everything with just 1 lens, then it must be the most versatile one ............... then the 18-135mm is suitable and the 18-200 VR is even more so because it has a wider zoom range and alos has VR.
However, the 18-200 VR is about $1,200.
Different focal lengths give different perspectives as the shooting distances will be different.
At the end of the day, only you will know which focal lengths and apertures you need for your type of photography.
For most/many people, landscape looks more dramatic and has more impact with a field of view much wider than 35mm, meaning shorter than 24mm lens on the D40.
For portraits, many people prefer to shoot with focal lengths of 50mm-85mm on a 1.5x crop DSLR like the D40 and with apetures smaller than F/2 in order to have a shallow Depth of field to isolate the person against an out-of-focus background.
Therefore for the best results, dedicated lenses for each type of photography is needed.
A verstatile zoom lens compromises in picture sharpness and apertures and therefore some people start to sell their versatile zoom lenses and buy dedicated lenses once they know the type of photography they want to achieve.