Writing a letter is a good way to go...You can write many drafts until you are sure you are clearly stating your intentions. Don't feel rushed at all, and once you are ready, and feel solid in the message you want to deliver, then do so. There are many things to focus on in the meantime. You can begin your transition, if that is what you want, without telling anyone until you are ready. Your family may or may not be supportive, but I would encourage you to be very solid in knowing who you are and how to proceed with your life before telling them. The more confidence you show in yourself and your decisions, the easier it may be for your loved ones to be accepting and/or supportive.
The person I thought would be most supportive, my sister, turned out to be the least, which is still a painful disappointment four years later. My mother, who I thought would truly not understand, has been great, and the day I told her, she asked me "What took you so long?" (I began transition at age 40) So you never really know what people will do or say or how they will show their love for you, but it is a lot easier for everyone if YOU have confidence in your decisions.
Take your time, become who you are, and then share that with your family. There is no time limit.