Check your spam email first, but then try going to https://courses.dts.edu to access your course.
If you signed up for a course, but never received an email to log you into the first lesson, check your spam folder first, but then try going to https://courses.dts.edu and clicking on the course you're trying to access.
The full list of courses is available at that link, so you'll need to find your specific course listed there.