To make full use of a sprint, you'll want to set estimated time or effort (depending on how you've configured your agile settings) on your tickets.
To set time on an existing ticket, navigate to it and click the Milestone/Dates tab. If you've set your project up to use time-based estimates, enter the Estimated Time Required:
Or if you've set up to use Story points, set the Estimated Effort Required:
Then just click Post Update to finish.