Follow the instructions for your operating system:
Additional instructions for specific products:
Solutions to common problems. If you're still having difficulty, ask in the forums.
Code and circuits for fundamental tasks.
Longer explanations of various aspects of the Arduino hardware and software.
Modifying and extending the Arduino hardware and software.
Links for further reading.
Description of the functions and keywords in the core Arduino programming language.
Extended Reference: includes advanced functions and language constructs.
Libraries: Additional functionality for particular hardware or uses.
Comparison: Comparisons with other programming languages.
A guide to the Arduino development environment.
Features and reference designs for the Arduino i/o boards:
Index: Complete history of the various versions of the Arduino board.
Additionally, see the community-maintained documentation on the playground wiki.