TinyMCE for comment textarea
First go to /admin/structure/types and edit your content type.
Click on the "Comment Fields" tab at the top of the page and then edit the comment field.
Here you can choose the text format that your comment field will use.

Now go to /admin/config/content/wysiwyg and choose the text editing you want to use for your text format. You can also edit any configuration setting your editor of choice here.