Wide Angle and Telephoto Lenses

dddodgerrr

Hello everyone,
After many years of using compacts I was given a D3100 as a present this year. It is my first DSLR, and I have started to teach myself how to get off of auto and onto manual through books and getting out with my camera (which I love).
My question is do you have any advice on wide angle and telephoto lenses? I have got a beginners course soon, which also covers these two areas.
A work colleague has suggested the Sigma 10mm-20mm and the Sigma 50mm-500mm. Any advice on whether these are of good quality and value for the money? I was originally looking only at Nikon lenses. My main areas of interest are landscapes, nature, night sky, abstract and architecture, if that is of any help.
Thank you.