I have Nikon D50 with kit lens. I want to buy some lenses to improve my pictures. Can anyone give me some tips to help me to choose suitable lenses for me? My main interest are landscapes and nature photography. Do you think a wide range zoom lens like sigma 18-200 are a good idea?
First off, additional lenses won't improve your pictures. They may give you different perspectives, but not necessarily "better" images.
I aay this over and over. It's good photographic techniques and a vision that will help you create a well recieved unified body of work. Good composition, proper exposure and using a tripod will give you the best images.
As for purchasing additional lenses, If it were me, I'd not go with that wide a range because the image quality probably wouldn't be as good as with a more limited range zoom. But if you're looking for a one lens to do it all, that may be the way to go.
I'll leave you with this thought. I once owned the Nikon 35mm - 200mm zoom lens and traveled to Austria with a two lens kit. The 35 - 200 and a 20 because they gave me a wide range with a common filter size. I came back with images that sold for years, but I used a tripod for every photo and lost whatever spontaneity I might have had with small faster single focal length lenses.