If you're seeing the seating chart tool for the very first time, you should see the screen below asking you how many tables you will have at your reception.
Once you've entered a number and press "Continue", you'll be taken to the seating chart tool with a list of tables and guests who have been automatically assigned to those tables to give you a starting point to work with.
From there, you can start to click and drag guests around to finalize your seating arrangements.
If you ever want to start over, you can click the "Reset tables" link to go back to the screen above.
Go here to learn how to print out your seating chart.