Anyone can edit Wikipedia. But with an account, you get more: your own username, a user page to share your interests, and a Talk page for discussions with editors, plus the ability to create new articles, edit semi-protected ones, and customize your preferences.
And you can set one up in, well sixty seconds: all you need is a username and password.
To start an account, click on "Create account" at the top right of any page. Here you'll have to pick a username. You can use your real name, but think carefully about it first. You'll also create a password and, if you want, provide an email. Click "Create your account". Done.
At first, the site will ask if you want to start editing a random article immediately. You can also read this page on getting started or take "The Wikipedia Adventure". Be bold and have fun. Happy editing!