Since I don't know anything about your skills with a database its hard for me to know where to start.
But when it comes to PHP you should definetely read articles about sessions. You can find some on this site.
Short about a session:
A session starts when you open a browser window and enters a website. The session last in this browser window until it times out or you close the browser window.
So what can a session do for you.
In this case you're making a shopping cart, so I suppose you want to have a shopping basket and items to put into it.
Think of a session as your "basket". In this basket you can store variables(in your case the variables will contain the items).
These variables will be available the entire session.
You can also remove items from your session.
Anyway, your options are many with sessions, so my advice is to look into this as soon as you can.
I could have written a lot more, but then this would turn into a tutorial. And as I said... they already exists.
