magick is energy found in nature. a spell uses this energy to create change in ones life. as a force of nature, it is limited by nature. a spell to bring love into your life will work, because you're changing the energy around you to attract love, while a spell to make objects fly won't because this isn't naturally possible for an object to levitate and fly by simply pointing at it. [sucks because i would love to stir my tea with my mind like in Practice Magic lol]
there's some beginners guide and some good articles on the site that could help you. you could also try looking at about.com and try to get a few books, specifically ones by Scott Cunningham or Raymond Buckland.