Attached to your admission letter you can find your personal Pre-Master's programme. There you can also find the information you need for enrolling in the courses you need to follow.
Most courses you can find in the Course Catalogue. For courses that are not given by Utrecht University you can follow the instructions in the admission letter. All information about beta4all courses you can find at www.beta4all.nl. For more information you can ask your Programme Coordinator (email@example.com).
You can start with your Master's Programme as soon as you have finished your Pre-Master's programme, not before! Please note you have to apply for your Master's programme again following the application procedure. Mind the deadlines!