please help me make Class Diagram for Cafeteria Website.

I have scenario for making cafeteria Website . My team want to increase the performance , so I will create a class diagram for this scenario. I read some books and information about how to write class diagram , but it is only for a software , I do not know how to apply it to a website project . How I can make this class diagram ? Please help me. Thanks

This is my scenario

LOGIN

Brief Description: This allows the customer or the admin to log in their homepages.
Initial Assumption: The users/admin have to access the webpage using the internet connection and browser.
Step-By-Step Description:
1. The user enters his/her username and password.
2. The system gets the input information, connect to the database and check the given login information, and confirm the information.
3. The System send a successful message to the interface.
4. The interface display the message in the user’s screen.
5. The System return the homepage to the user.
What Can Go Wrong: If the user inputs the wrong usernames/password , the system will return a unsuccessful message (“Wrong username or passwords , Please input again”), and user input again .
Other Activities: None
System State on Complete: The System can identify the user and allow the user to access their pages.

Register

Brief Description: This allows the customer register an account
Initial Assumption: The users have to access the webpage using the internet connection and browser.
Step-By-Step Description:
1. The user enters his/her username and password, Full-Name , student ID or Staff ID, Mobile numbers, email, …
2. The system gets the input information, connect to the database, update the database.
3. The System send a successful message to the interface.
4. The interface display the completed register-form in the user’s screen.
What Can Go Wrong: If the user inputs the already used usernames, the system will return a unsuccessful message (“username already used, Please input again”), and user input again register-form.
Other Activities: None
System State on Complete: The System can create an new account for the new customer to manage users. The customer use the usernames/password for the login pages.

Select the food/drinks:

Brief Description: This allows the customer select the food/drink in the cafeteria shop
Initial Assumption: The users have to access the webpage using the internet connection and browser.
Step-By-Step Description:
1. The user open the webpage.
2. Select the food/drink on the webpage to buy by click the “Add-to-Cart” button.
3. The system will direct the user to the “Add-to-Cart” page.
What Can Go Wrong: If the user chooses “not available item”, the system will send a message:”not available item”.
Other Activities: If the user want to buy other food/drink , they can selected others and also click “Add-to-Cart” again ,
System State on Complete: The “Add-to-Cart” page will return the user interface.

CHECKOUT THE PRODUCT

Brief Description: This allows the users check out the selected product
Initial Assumption:
1. The user has logged in his/her homepage.
2. The user has selected the product to check out.
Step-By-Step Description:
1. The user completed the “add-to-cart” page, after that click on the “Check out” button to pay total fees.
2. The System will check the user’s credit balance to ensure that the user has enough credits to buy the product.
3. The system will confirm the checkout request from the user and write the information into the database.
4. The system displays the message to announce that he/she has checked out the product successfully.
What Can Go Wrong: If the user’s account balance has not enough credits to buy the product, the system will display the checkout error message.
Other Activities: None
System State on Complete: The check-out process is completed successfully.

ADDING FOOD/DRINK

Brief Description: This allows the admin to add the new food/drink
Initial Assumption: the admin must be login by admin’s usernames and password
Step-By-Step Description:
1. Admin log in to the system by usernames and password. The system will check , if the usernames and password is right , the system will go to the “admin manager’s page”
2. The admin will choose “menu” button in the admin manager page, choose “add new food/drink”
3. The admin input the information of the new food/drink
4. The admin send an adding request to the system by choosing “update” button
5. The system will update the information into the database and displays the successful message.
What Can Go Wrong: None
Other Activities: None
System State on Complete: The database updated the new drink/food.

UPDATING FOOD/DRINK INFORMATION

Brief Description: This allows the admin to update the information of the store
Initial Assumption: the admin must be login by admin’s usernames and password
Step-By-Step Description:
6. Admin log in to the system by usernames and password . The system will check , if the usernames and password is right , the system will go to the “admin manager’s page”
7. The admin will choose “menu” button in the admin manager page, edit the information.
8. The admin send an update request to the system by choosing “update” button
9. The system will update the information and displays the successful message.
What Can Go Wrong: None
Other Activities: None
System State on Complete: The database updated the new information of the drink/food.

DELETE FOOD/DRINK

Brief Description: This allows the admin to delete food/drink
Initial Assumption: the admin must be login by admin’s usernames and password
Step-By-Step Description:
1. Admin log in to the system by usernames and password . The system will check , if the usernames and password is right , the system will go to the “admin manager’s page”
2. The admin will choose “menu” button in the admin manager page, choose the food/drink that the admin want to remove.
3. The admin click on “delete” button.
4. The admin send an remove-items request to the system. The system will remove food/drink information in the database.
5. The system will update the information and displays the successful message.
What Can Go Wrong: None
Other Activities: None
System State on Complete: The database updated the new information of the drink/food.

VIEW REPORT

Brief Description: This allows the admin to view the report
Initial Assumption: the admin must be login by admin’s usernames and password
Step-By-Step Description:
1. Admin log in to the system by usernames and password. The system will check , if the usernames and password is right , the system will go to the “admin manager’s page”
2. The admin will choose “report” button in the admin manager page.
3. The system will return the report page to the interface.
What Can Go Wrong: None
Other Activities: None
System State on Complete: the report page will be returned to the admin.