how to retrieve a table in a string (could be JSON) ?

Hello,
I've a web site, that display some data with dataTables.
The user can filter those data to do a subset selection.
My problem is, by clicking on a button (for example), to extract the filtered data (in fact the first column) to do a php treatment. If I can obtain those data, I can then use an Ajax script, it's not a problem. My problem, is how do the extraction ?
For example, simply, clicking on a button, how can I have an alert box with those data ?
Any tutorial is welcome !

Thank a lot,

Sohnic