subroutines in scripts

Hi friends,

can any one tell me hw to call subroutines in scripts for:

Find Invoice amount based on:

REGUP-BUKRS (Company code) = BSAK-BUKRS

REGUP-BELNR (Document No) = BSAK-BELNR

REGUP-GJAHR (fiscal year) = BSAK-BELNR

REGUP-BUZEI (line item id) = BSAK-BUZEI

and the table bsak-wrbtr...

wot to write in forms n what to write in form n where???

can ant one help me plz..very urgent................