Once you sign up for a course and activate your account, click on the Sign In button in the upper right corner of the edx.org home page, and type in your email address and edX password. This will take you to your edX dashboard, with access to each of your active courses.
The course start dates are listed on your dashboard. After the course begins, you will see a "View course" button. This button will take you into the course itself. Next, look to the top left and you'll see a Course tab. The Course tab will have the assignments and instructions needed to begin.
(Before a course begins, the course will be listed on your dashboard but will not yet have a “view course” option.)