Name, baptism date, and ward and stake you reside in. If you don't know your ward and stake because you've moved or haven't been an active member for a long time, you can send your letter directly to Church headquarters and they will be able to forward a copy of your resignation to your local Church authorities. In your letter you should state that you have resigned your membership as of that date and request your name be removed from the records of the Church.
You may state you understand the consequences of name removal include cancelation of all temple blessings and church ordinances. You may request that you want the 30-day recision period waived and to process the request immediately, but whether or not that request is honored is up to the bishop and stake president.
The preceding is basically all that is required in the letter to have your name removed. Some people who leave the Church go to great lengths to explain all the reasons they no longer believe Mormonism is true in order to make an impact on leaders receiving the letter. Others just simply send a concise request. To avoid LDS leaders drawing the process out as long as possible, it is recommended you either hand-deliver the letter to your bishop and stake president or send a certified letter through the mail. In addition, send a copy directly to:
Member and Statistical Records Department
50 East North Temple St
Salt Lake City, UT 84150-5310
Be sure to "CC:" the bottom of your letter to your local leaders that you are sending a copy directly to LDS Church headquesters. This will encourage them to not drag their feet.
Writing and sending your resignation letter is probably the most intimidating or difficult part of leaving the Church initially, at least as far as formalities go. However, it is an important step in freeing yourself from Mormonism's hold, both spiritually and psychologically. It will also put a halt to having home teachers or visiting teachers make their monthly rounds to your home.
For assistance in dealing with the other aspects of leaving the Church; emotional issues, relationships, and such, consider joining a support group. Click Here for more info.
