You need to have a user account for many common tasks on Wikipedia, such as creating new articles, uploading images, keeping a watchlist to monitor changes, and so on. A full list of the benefits can be found here. This tutorial describes the three simple steps to get an account on Wikipedia.
Before starting, you should think of a username and password. Wikipedia has a few guidelines on choosing a username, and some suggestions on choosing a secure password. This tutorial covers some key issues you should consider.
A video has been made of this tutorial.