Workflow for CRM.

Dear Experts,

I am a beginer to Workflow.

I have a scenario in CRM.

1,The requester logs into portal and creates an activity in CRM PCUI screen.

what i want to do in workflow is

When the requester creates the activity and saves their is a field with a user name.

I want to send a mail to that user name.

and after that the user will acknowledge it, then the requester should get a confirming mail.

After the user completes the task and comes back to the Portal and changes the status to completed.

then the workflow should send a mail to the requester stating that task is completed.

I will be glad if somebody can explain me the steps involved in doing this workflow.

thanks and regards

Neel