Start first at a comfortable level of speed. Often we find that the biggest problem with people's accent is that they want to intonate faster than they can think about what they are going to say. Therefore, what you can do is create statements and sentences that you feel comfortable repeating and including in everyday conversations (usually these are statements about yourself) and practice them over and over as much as you can.
In the same token, take the time to write down these same statements over and over as a way to connect the sounds to the symbols. Usually the LAST part of language acquisition that is measured though the process is the production of spoken language. Therefore, practice by talking first, and then make a habit of reading and writing.
Business communication often has its own "vocabulary," style, and forms. So examples of business communications, often available on-line, might be helpful. Or, if you know someone who writes and receives business communications on a regular basis, you could look over some of that person's materials, be able to ask questions, and get some good answers.
With anything you want to improve, PRACTICE is the key. The more immersed you are in a language, the easier it will get to choose the correct word, sentence structure, verb tense, etc. So, listen to English radio, watch English TV, read English magazines and books, and practice oral speaking as much as you are able.
When I was in South Korea, I saught children to befriend. They were patient, didn't mind correcting me or repeating something multiple times, and we laughed a lot with each other. They also don't seem to be under the same time restraints as adults. They taught me Korean, and I spoke with them in English as well...win-win. You might try that as well.
Reading, writing, speaking, and listening are the only ways to learn or improve language skills. It's easier for children to learn a language because they have both a need to learn and a natural aptitude for the sounds of speech. Your need to learn is just as great, so you'll have to compensate for the diminished aptitude by practicing. You have plenty of good ideas, here already; I would add going to places where you can hear real people speaking the language in useful and productive ways.
I would suggest finding a book or books that have been translated into your native language from English (or vice versa). Read the books in English and try to translate them into your native language. Compare your translation with the official one. This can help you determine if you are understanding the nuances of various English expressions.
You already received an excellent short lesson in the post by missy above. I think the very best way for you to improve your written and verbal skills is to read as much as you can, and converse in English as much as possible. As suggested above, don't be afraid to ask for advice or suggestions. In addition to any school work you may have in English, try and find yourself some reading material that interests you, and read it as a supplement to your class assignments. Magazines are great if you feel overwhelmed with more complicated novels or short stories.
The more you read in English, the more you will begin to notice common language structures in English. For example, in the original post, you wrote how to improved english. Whenever you write the word 'to' and follow it with a verb, it will be in the verb's base form. In other words, you will not add the ed or ing to the verb. It should just be how to improve English.
The more you speak with native English speakers, the more you will have practice learning to speak with improved fluency. Allow people to correct you when you speak. Ask for the rule too.