Improving your English quickly will mean a lot of practice, but it should be focused on the vocabulary you need at the beginning. Typically, you would find a business English teacher who has experience of sales terminology, and a good understanding of international trade. You will also need a textbook with English vocabulary to study.
Your teacher will help you with reading, writing, listening, and speaking, and for your needs, this should be intensive for a few weeks.
Best wishes, Andi