Hi Erica. If you're starting from scratch then buying the Sigma 10-20mm is a good move especially for people shots, I have the f3.5 version and it's a cracker. I also have an 18-200mm which is a fine lens for most other types of photography and the APSC sensor in my camera means that it equates to a 27-300mm in film terms which gives a really coverage. For birds and wildlife and sport, I use the Sigma 120-400mm and again the sensor dictates that this will be like a 180-600mm, so this is a cracking lens also. For most purposes though, my 18-200mm stays on the camera. All the best to you. Mike.
Humility is an endearing quality and gains many friends, whereas arrogance loses them. Mike, 2012