To create a new Sprint with new tickets, click the Build Backlog button to the top right of your sprint management page:
Adding tickets manually
You'll see an empty list of ticket fields where you can type a description for each ticket and set a priority. Add you information in for each ticket and select their priorities, then click Create tickets and add to backlog.
Adding tickets via CSV
Alternatively, you can add tickets en-masse using a CSV file, just locate the appropriate file on your computer after clicking the Choose File button, then click Upload to finish.
You'll need to ensure your CSV file follows the format specified above.