Click the top box to give the assignment a name

Within the 'Assignment Part 1' category are the dates for the assignment. Start date determines when the assignment opens to students, due date is the date that the assignment is due, and post date is the date an assignment has to be posted by

Ensure that your options are similar to this. You MUST have at least one of the three checks be set to 'Yes', otherwise a similarity report will NOT be generated for any submission to an assignment. 

Creating a Turnitin Assignment