How do I fill in form with user's info if they are already logged in?

I have an online order form, where part of the order process is to create a user account.

If that customer is already logged in, how can I hide the account creation fields, or pre-fill those fields based on the user that is logged in?

For example, when a user places their first order, I want them to create a username and password. This is working as expected. But if they are logged in already and want to order more services, I need a way for their account info to show up in the Account creation fields, or just hide those fields all together.

Also, in another example, if someone is placing an order when not logged in, and they already have an account but not logged in, how can I have it prompt for their login before continuing if they set a field called "are you a current customer?" field? Or, if they enter an email address already in use, it would prompt them to login?