Welcome to the forums.
I have underlined and given alternatives to your original as the constant use of "I" can give a bad impression.Dear Ms xxx xxx
Application for the position of xxx Marketing Assistant
I was interested to see your advertisement for the position of xxx Marketing Assistant on the homepage of the xxx-Schule.
I finished my education as a foreign language correspondence clerk on this year’s July.
During this period of time I gained a good command of English, especially in the area of correspondence. Furthermore, I could improve improved my already good IT user skills (MS-Office applications).
I am interested in the work of a xxx Marketing Assistant because I would have it would give me the opportunity to realise my organisational expertise and develop it. I am also interested in marketing and promotional activities. I proved to be flexible, reliable and My flexibility and reliability during my previoussideline omit jobs show that I am a team player who is able to work under pressure .
I am hoping to apply my expertise and to demonstrate my dedication to your company. I would be satisfied with the invitation for an interview at which I could introduce myself.
For you information, I am enclosing/attaching my cv. I hope that it will be of interest, and that you will feel my expertise and potential will benefit your company. I am available for interview at any time, and can be contacted [give email address and telephone number(s). I look forward to hearing from you.