Working With Database Outputs

I am establishing a connection with a database and by using a query generating output(extracting from the database). On every line of output, there is a checkbox. I want the user to select which ever output he/she wants(via the check box) and press enter at the bottom. This is working fine.

On the next page I want the page to only show the selected output(selecxted by the checkbox on the first page).

My questions.......

1) How do I count dynamic output coming out of the database?
2) After the user selects.......how do I know which line he/she selected(in order to reproduce on the second page)?
3) How do I put those values on a session variable(the database output)?

What I am trying to do is......basically create a shopping cart, which lists the item, then calculates the price on the item........and so on and so forth. I am badly stuck on the first step.