Dear Sir/Madam,
This application is in regards to your job offer. I consider myself suitable for this position as I am fluent in English (have passed my state examination with flying colors). I could help improve your grammar, vocabulary and overall fluency, recommend movies or books to improve your skills (we could discuss them later), whatever helps you become better.
Do not hesitate to contact me if you need any further information.
Yours sincerely,
Rosita Steponavičiūtė.