How can I add a RichText-Field to my plugin's CP-section?
To do this you'll need to take a few steps:


Load the necessary resources which include the redactor.css and redactor.js file.  And if you are using Craft's macros for your fields, the form macro file.

{% includeCssResource "lib/redactor/redactor.css" %}
{% includeJsResource "lib/redactor/redactor.js" %}

{% import "_includes/forms" as forms %}

Add a class to your field that you will use to initialize your Redactor toolbar.  Here's a simplified textarea macro:

{{ forms.textareaField({
    class: 'myFieldsRedactorClass',
    name: name,
    value: (value is defined) ? value : ''
}) }}

Initialize the redactor field type with your desired toolbar.

(function() {
    $('.myFieldsRedactorClass').redactor({
        buttons: ['bold', 'italic'] 
    });
})();



Here's a related article on customizing Redactor toolbars from Straight Up Craft:
Custom Redactor Toolbars for the Rich Text Field in Craft CMS

Note: 
One thing to be mindful of, which I'm not sure what to say about, is that if you have other Rich Text fields in your section with you custom field type, Craft is probably already loading these resources. I don't know if there is some way to check to see if these are loaded and only load the redactor css and js assets if they are not being loaded elsewhere.