This is not something you are going to learn overnight my friend!
You are going to need to study ASP.NET to learn how to get the data from the user form, authentication methods and cookies etc..
You are going to need to study SSL to learn how to retreive data SECURELY from the user (you mention payments)
You are going to need to study Webservices to process the payment (I advise finding a host that does this for you, I use pipeten.com)
You are going to need to study ADO.NET to put the data in/get data from your database.
You don't need to send the password in the email but you will want to send a URL with a GUID in the query string for them to click that you saved earlier against their created account in your DB.
You need to study the System.Web.Mail namespace in .NET to learn how to send email with .NET
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since Feb 2005