Wikipedia: Editing and Contributing

1. Go to Wikipedia's main page.
2. In the top, right hand corner, click create account.
3. Create a username and password. For more information on usernames, see Wikipedia's username policy.
4. Enter your email, if desired.
5. Complete the CAPTCHA security check.
6. Click create your account, and you are ready to edit!

"You don't need to be registered to contribute, but getting an account allows you to:

• Choose an appropriate username, which will be reserved just for you on Wikipedia and other Wikimedia projects.
• Create your own user page, to collaborate, share information about yourself, or just practice editing and publishing in your own sandbox.
• Communicate with other editors via your own talk page. You can also opt in to exchanging emails with other users.
• Make it easier for other editors to communicate with you. Even if you don't feel any need to contact other editors, they may wish to contact you, which may be difficult if your IP address changes. Also, even if you have a static IP address so that it doesn't change, other editors may not know that is the case, so they may be deterred from posting to you on your IP talk page.
• Get automatic notifications that alert you when someone has {{ping}}ed you, or wikilinked your user ID.
• Use custom preferences to change the look and behaviour of Wikipedia.
• View a convenient list of all your contributions (edits), and use your watchlist to monitor changes made to pages that interest you.
• Use your unified login to work on Wikimedia's other projects, such as Wiktionary and Wikimedia Commons.
• Use more advanced editing tools.
• Edit without revealing your IP address (which can be used to trace your physical location) to the public.
• Vote for the Picture of the Year and members of the Wikimedia Board."

--Information from Wikipedia