Write "Dear Soldier" at the top of the page. If you know the name of the soldier you are sending the letter to, write their name instead of "soldier." Also include their rank.
Introduce yourself. Tell the soldier some things about you and why you are writing the letter.
Ask some questions, but be careful not to get too personal or ask anything inappropriate. Remember that they might not be able to answer some questions depending on where they are or what they are doing. Don't worry; they won't be mad if you ask the wrong thing on accident.
Write your last paragraph, thanking them for their time and letting them know that you will be excitedly waiting for a response. Also thank them for their service of keeping the country safe.
Write "Sincerely," at the bottom of the page leaving enough room for your signature.
Sign your name. If writing via email, just type your name.