First, click on the course that you want to add a Prerequisite to. Next, click on the General Settings tab in the course.
Next, scroll down to Prerequisites. Here, you'll be able to specify how many Credits a user must have in order to access the course, together with what courses a user needs to have passed. The two settings are independent and do not require the other to work.
If you click on Set Prerequisites, all you need to do is use the drop-down menu that appears to select the course that will act as the prerequisite. A course can have more than one prerequisite, so don't worry!