If your event has waves, you or your Team Captain chose a preferred wave when you entered.
You will probably get your first choice
Most people do. You may be moved, though, so that racers are spread evenly across the waves and none of them becomes overcrowded.
When it is confirmed
Your final wave is confirmed by your event organiser shortly before the event, usually one to two weeks before. You'll receive an email confirming your wave and start time.
Checking before then
Log in to your Racer Dashboard and look at My Entries. The wave currently assigned shows under the ticket. Bear in mind this can still change until the confirmation email arrives.
If you are in a team
Teams are kept together, so if you are moved your team moves with you. The exception is if members of your team are doing different distances, in which case you may start at different times.
If your wave looks wrong
Wave allocation is the event organiser's decision rather than Race Space's, so contact the organiser directly.