Under Setup > Reservations click on Charges. To create a charge click the Add Charge button. If there are existing charges you will see them listed in the table. To edit existing charges click on the Edit button that corresponds with the charge you wish to modify.
After clicking the Add Charge button you will be taken to the screen below. Enter in the name and description you would like for the charge as well as the value. You then have the option to assign an existing tax code that you have created (See Tax Codes) and also you can set it to a nightly charge or include it in the discount. Lastly you can choose if the charge is Active. If the charge is not active you will not see the charge when you attempt to add a charge to a reservation. Click the Save Charge button to save and return to the main charges screen.