You can find numerous free and premium plugins that will make your wordpress site multi author site. However, you can also convert your single user site into multi author site without using any plugins. Here is the step by step process.
Log into your wordpress dashboard. On the admin ribbon on the top hover over New (the option that lets you create new posts). Choose "user" option. Fill the form. You have to be careful about choosing the options for user role. If you choose administration, the new user will have access to your admin panel. You might not want to do this because your website can be hijacked. If you want the user just to post articles, choose author option. Hit save user. After that send login details to the user.
You can also access "add new user" option directly by choosing user option on the left hand side on your wordpress site.