How to grab the initial field in driver program ?

Hi,

In my SAP Driver program I am declaring all the tables from where the data will be fetched.

Now all these data selection depends on order number AUFNR field which I use to go in transaction CO03 or CO04.

I am just struggling and wondering how can my driver program fetch this order number field (AUFNR) on which all other data selection depends ?

Points will be awarded..

Tushar.