Hi and welcome to Using English.
The best way to build your vocabulary is to read. If you need to write business letters, then read business-related magazines and articles. When you come across a word you don't know, look it up. Put a check mark next to it in the dictionary. If you find you are looking up the same word more than once, it's time to really learn that word. Write sentences using it to practice. You can post them there.
Don't worry -- even experience writers will stare at the computer screen, struggling to come up with the right phrasing.