A multi-users system prblem

Hi,
 
I have created a webpage where users can select from a list of items then pay for them online, BUT the problem is that I'm using Arraylist to save the selected items for the user to pay for them and if more then one person is using the system at the same time the system will add the items of one user to another and will show the user items that he/she didn't picked.
So, how can I make my system a multi-user system so every user will get his/her own session to work on?
 
Thanks 
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