How to Post Customer invoice and Incoming Payment?

Hi,

Please give me some inputs for the below scenario. Please help me where i can do this..I could not find the T.Codes..

1. Customer Invoice

2. Customer Payment at Desk

3. Down payment

4. Down Payment Clearing

I could able to find the t.code for Down payment, But i could not find the T.Codes for Invoice and Incoming Payment and Payment Drawer.

Please help me..its bit urgent...