How do I create my autoincrement number?

My Table

main_index (AutoNumber),
payee_id (Number),
main_amount (Currency),
main_receipt (Number)

if my Last Receipt Number was 500, I would like to update my main_receipt
with the next number
501, 502, 503, etc, etc, according th main_idex field.


Thank you all,

Bre-x