A Letter to My Friend

Hi, John, 
How is your vacation going?, I suppose you must be having a wonderful time with your family. I am awfully sorry, but I have to give  for telling you some bad news.
Last night, when I was using your new laptop computer, my son spilled a cup of coffee was poured into onto the keyboard by my little boy. This has resulted in some of the keys not working properly, and may well bring about some other problems to the computer as well.
Without doubt, the keyboard should be replaced, on the whole. and there might can be some other more damages to the computer. What I mean is, the cost, I suppose, will is not merely be for just a keyboard.
I will take it to a repair shop as soon as possible. The first thing I need to do is to buy a new keyboard. At the same time, I will ask them to give the computer a thorough checkup to make sure whether everything is fine. Of course, I will bear all the expenditure of the repair.

I agree with Leigh's corrections. Only, for a letter to your friend, the last line sounds very stiff: Of course, I will bear all the expenditure of the repair.

I'd rather say something like: "Of course, I will pay for all needed repairs. Again, I'm so very sorry that this has happened!"

And definitely, the greeting in letters should be

Dear John,

Dear Mike,

etc.

A comma after the person's name, and then start your letter on the next line, or even skip a line.