It's about the fifth question in the FAQs (press link at top of every page or you can go direct here http://www.raspberrypi.org/faqs
The grammar of the answer can easily be misread so I'll give you a summary (of my understanding) but encourage you to read the rest of the faqs.
1. A Raspberry Pi
2. A Quick Start How to Connect it Guide and statutory declarations etc.
Everything else you'll need to buy separately.