Twig - render a rich text field
If you want a rich text field on a front end - this question has been answered in detail here:
How do I enable redactor rich text editor in a front-end form?

In short technically possible but not recommended.

If you want a rich text field for a back end, i.e. plugin, form:

The craft macros for form fields live in craft/app/templates/forms - however I do not believe there is a macro for a rich text field.  There are text and textarea macros you can use.

However, I think it is possible in a plugin - this apparently creates a rich text field widget in your craft dashboard, so you should be able to pinch the relevant code from: https://github.com/gilleard/RichTextWidget/blob/master/richtextwidget/widgets/RichTextWidget_DashboardWidget.php

(Specifically look at public function getSettingsHtml() I think)