The three pieces of advice I would offer a new employee I am supervising that has trouble with his/her written and oral communication are #1 – You need to seek help from your coworkers. In the short term I would suggest you ask one of your colleagues to proofread your work when needed.
#2 – Your long term goal should be to get professional help. Short term help from a co-worker will not solve the problem, it is only a start. A continuing education class will help to make long term, lasting changes.
#3 – None of us are perfect, we all have strengths and weaknesses so don’t feel ashamed. I think this is very important to say because there is a perception of unintelligence associated with imperfect grammar and speech. Too often people forget that we are all a product of our upbringing, education, and environments, some of which we have no control over. Therefore, you may not have had the privilege of attending private schools and getting a first rate education. Also, English may not be your first language.
Speech and written communications are subjective. After all, there is the perception by many (especially Brits) that Americans have butchered the Kings English.