Let's say you want to take the networking major. At enrolment, you'll probably enrol in:
FIRST YEAR
Autumn semester
Communication for IT Professionals
Introduction to Information Systems
Programming Fundamentals
Web Systems
Spring semester
Business Requirements Modelling
Networking Essentials
Applications Programming
Elective Subject
RED: Core subjects
BLUE: MAJOR
GREEN: Elective OR One subject from sub-major OR One subject from 2nd Major
No, you do not have to choose everything by/at enrolment, but you will have to know which major you want to do and at least one elective or at least one sub-major. You do not have to know all electives or all sub-majors you want to take.
Side note: It's not recommended you take two majors, but if you don't want to do any electives or any sub-majors, you'll have to know both majors, since at enrolment you will be enrolling for the entire first year.