1) Timetabling day. They will show you various time slots available for lectures or tutorials that you can manually select. There is also the option of "generate a clash free timetable", this option however may give you bad times. Keep in mind that each time slot for lectures can only allow for a certain amount of students, so they may fill up quickly. Since you're a first year, many of the subjects have "staggered release" where there will be a later date to choose certain time slots that have previously been full. Also, keep in mind that the portal (the site you timetable on) may be extremely slow due to the traffic.
2) Creating a mock timetable enables you to create the best possible timetable to program on Monday. Classes can fill up quickly so it's good to know exactly what times you are choosing for lectures etc.
3) Create a mock timetable (via the 2013 university handbook)