[otrs] Customize "Compose answer (email)" form

Dears, I want to customize the form "Compose answer (email)". Even though I've just created the response for certain queue, when agents compose answer to customers the text fox is filled by default texts besides the texts that are written by me. So I want to delete these default texts. Also I want to insert customers email address into "To" field automatically. How can I do these? I did not find source code of that form. -- Altangerel Ganbold