There are a couple of different types of letters. It is important to understand who you're writing to and what type of letter you should draft before getting started. If you're writing to someone you know professionally, a business, or a government official or department you'll be composing a formal letter which has different requirements from an informal letter. You can draft an email if the subject is urgent or if the person you're writing prefers to correspond via electronic mail. An informal letter will be written to someone you know like a friend or a family member. The composition of an informal letter is more relaxed than a formal letter.
A formal letter will need to be typed and printed unless you're using email. Utilize common software like Open Office, Text Edit, or Google Docs. Each of these programs offers an introductory lesson on how to generally use the software. Remember that if you're writing to someone whom you work with currently, the tone of your letter doesn't have to stick to severely formal layout for a letter. Because you know your boss or co-worker your letter can be more relaxed in tone.