If you're doing the general IELTS test, try this writing task 1 question from Cambridge book 9, page 117:
This should be a formal letter, so start with "Dear Mr. Smith" (Mr. or Mrs. and any surname). Then write a short paragraph for each of the three bullet points in the task box above. End the letter with "Yours sincerely" and a full name (you don't need to use your own name).
It might help if you do a quick plan before you start writing. Just spend a couple of minutes thinking of ideas for each bullet point.