我想申请调部门，但要用英文写申请书，我英文不好，拜托各位帮帮忙~

Dear Mr./ Ms. 或者用To it whom may concerns,
I have been here for 3 months and getting along with the new colleagues. With the practised months, I have learnt more and can be competent for the job. First of all, I would like to take the opportunity to thank all of the support and great help from the department. Secondly, I am considering transfer to marketing department due to the new challenge and set up high personal goal, I will try to improve my English during spare time in order to be more effective with the new position.
I am very much delighted to work here and hopefully I could do more contribution in the future.
Thank you.
说明：我尽量用了你的原意，但有些中文表达在英语中是不一样的。商业英语在表达上更是有很大的区别。希望以上对你有帮助，学习英语需要日积月累，加油！