In the Test Runs tab click the "Create Test Run +" button, you will then be prompted to input the information in the name, description, Milestone, priority, & Tags sections.
When finished click "Create Test Run" to finish, or click the "Actions" button to display a menu where you can add Test Cases or duplicate Test Runs.
You have now created a Test Run! The next few pages will explain how to add Test Cases and duplicate Test Runs. Click "Next"