Identifying which user posts a form

Identifying which user posts a form

I cannot find a way to capture the logged in user that is submitting a form. How can I add the users name and/or email address to the form automatically (i.e they don’t have to select their own name or email address). The reason for this is I want to capture which user is submitting the form based on their login, otherwise a user could add anyone’s email address and name to a form.