A test can be anywhere in a course, so your instructor will let you know where it's located via email, announcement or message. This example will go through the Course Menu.
Make sure that you always log out of Blackboard. If you have more than one active session of Blackboard running (even if you completely closed the window, the session is likely still active) it could kick you out of the test or not save your answers, making for a mighty annoying headache that both you and your instructor will both have to wrangle with.
If you are taking a test on a Mac you need to be sure that your browser settings have their font size settings at 14 or higher, so the Save button for test questions will not be covered by (or covering) some of the exam question text. Safari > Preferences > Advanced > Accessibility