When writing a letter to a friend you can start with greetings and asking about his well being. Then you can tell him of your plans for the summer and ask him if he would like to join you. Lastly, you can give him any additional details on what to carry and where you are going to stay.
Well... If you want to write a letter you will need a pen and paper. Write that you want him to visit and include your name and contact information. Visit www.letters.com for more information. Hope this helps.