You can wear the watch on the wrist that is most comfortable. For the best accuracy, you will specify whether you wear your Fitbit on your dominant or non-dominant hand. Your dominant hand is the one you use for writing and eating. The Fitbit defaults to assume the watch is worn on the non-dominant hand. If you prefer to wear it on the dominant hand, change the wrist setting in the Fitbit app:

From the ‘Today’ tab in the Fitbit app, tap your profile picture > Sense tile > Wrist > Dominant.