The most common problem which one faces during the very early stage of SAP Implementation, is how to load the existing data into SAP from the existing legacy systems.
The simple solution is use the SAP's standard Data Transfer Workbench (tr SXDA).
It gives you the option to load data of almost all Business Functions. TO assist in this process it also provides you the option to generate sample File Format (with data or only structure) to load, in different languages viz COBOL, C, ABAP etc.
Transation Code: SXDA