work flow object type for purchase requisition

helllo

I have a requirement in which i have to create a custom table with the fields plant,storage location and approver user id.........and when the purchase requisition is created the work flow should be triggered and the fields issuing plant and storage location should be read frm the PR and the related approver user id should be sent a mail for approval and .........and also the requisitioner and creator should be sent a mail when rejection or approval....now my question is where can i get the approver user id frm????how to send the mail to them....could anybody please help me with this requirement briefly even if it takes some time

regards

anik