Welcome aboard! We're glad you wish to help develop Wikipedia, and we hope you enjoy participating in this virtual community as much as we do. As a new contributor, you may feel a little overwhelmed by the sheer size and scope of this project (which includes over 100,000 regular participants)...
...but don't worry, things will become easier and clearer as you go, and if you have any problems, there will always be other Wikipedians happy to help, see Wikipedia:Requests for more information. There are some resources set forth below to help you get started. Good luck and have fun!
To browse help related pages see the help menu or help directory. Or you can use the search box at Help:contents (accessed at any time by clicking Help displayed on the left side of all pages). There is also a copy of a comprehensive published book, at Wikipedia: The Missing Manual.
Tip: Sign your name - At the end of your comments simply type four tildes (~), like this: ~~~~. If you are using the edit toolbar option (it usually appears above the edit screen by default), click the signature icon: , to add the four tildes; this will automatically produce your username and the time, aiding the flow of discussion.