If you control your own WordPress installation, you can create and manage accounts in the system by clicking the Users tab. Each new user can be given the role of Contributor, Author, Editor, or Administrator. The exact capabilities of these roles are described here. Don't give new users any more privileges than they need!

If you've been asked to blog for a publication, your web administrator will create the account for you, and provide an initial password. Your first act after initial login should be to reset your password. Click on your name in the upper right corner, then scroll to the bottom of the Profile page to change your password.