automating password generation

I'm putting in a member's area with login & password on my site. It works like this:
click 'join now' sends you to payment centre. Transaction completed sends an email to me with transaction & contact details and also one to the customer. currently I contact the customer manually giving details of password & login.

Where can I get information on putting in a fully automated system so that the customer doesn't have to wait for the info and I am taken out of the loop? With thanks...