Ask HN: What's the best way to process a form?

Once you have a form in your webpage, and a user clicks "submit", the information goes to your server, right? My question is, how do i process that information, so that for example the user can register with a username and password and then login? What language should i learn? I know SQL is the basics of keeping the information of users in a database at your server, but is it what i need to also provide the login feature?