This is an excellent resource for those unsure, confused or hostile to the idea that human activities are dangerously disrupting the climate. John Cook, with the help of a number of climate scientists and other helpers, has put together a free document titled, The Scientific Guide to Global Warming Skepticism (pdf here). It is well illustrated, clearly written, brief and to the point, addressing some of the most common queries and misunderstandings.
If your question isn't covered by the guide, the rest of John's site, Skeptical Science, has responses to more than one hundred common arguments concerning climate change (with plenty of links to published peer-reviewed scientific papers, or just short simple answers for beginners). I have recommended this excellent and highly informative website before, but let me do so again.