As aidanodr said, it's difficult to answer without knowing if you're using an open source app or writing your own tools to accomplish the task, you also don't mention what technologies you're using (if you're creating your own system).
Assuming you're going 100% custom, I can tell you that one of the easiest things to do when making an ecommerce site is facilitating user login/registration.
Without going into specifics: When a user registers, all you're doing is creating rows in different database tables (i.e. a new row in the customers and cart table). The rows in the different tables would be linked by a field that you might call CustomerID or something similar. When the user logs in, you'd simply write code to search through the customers table until you find a record that matches user input.
I have created a .NET guestbook application that demonstrates the most basic applications of these concepts. You can download it here:
http://code.msdn.com/netguest
You will need a microsoft web server and SQL Server to install it on to have a look though (I would assume the learning institution where you're studying has these tools somewhere).
If you're not writing your own app, but instead using some type of downloadable software, I would recommend going through the software's documentation or checking out their website to find your answer.