First change the age to say 15-16 thats really low for a survivor, I think a twelve year old could never stand with the struggles the main character goes through. Its not a very realistic age of a survivor.
Second, like said above try to vary the word use in your text.
Third, The problem I have occoured through your text is that you do not describe the settings good enough, heres an example.
"He said in a faint voice, stuttering and struggling to speak. I began to cry with tears rolling down my cheeks.
It was hot... Very hot.My mom woke up and said with a half burnt face..."
First thing about this little piece is that "IT WAS HOT... VERY HOT" it dosent describe the setting enough for the reader to really visualize the surroundings, its barely enough to visualise the setting.
And another thing about that little piece is that if the main character sees his mom and dad under a pile of rubble or whatever, is it most important for you to describe that the character thinks its hot, or the fear within him when entering a burning collapsing house, or the fact that he sees his parents half dead lying under a flaming piece of un-moveable rubble?!
And I am kind of wondering over this line.
"That's when I saw-- I don't know what I saw..."
Try to describe the zombie a bit here, Its to give the reader a clue about whats happening(its more exiting that way). That line you have thrown in is to cheap, you immediately understand that its not human, you have to build up the tension before the climax.
"We stopped at the U.S Army National Guard Military base. I was happy to see a building that is not in rubble. I walked inside and saw many people with weapons.
-Is he bitten? A man in a white robe asks..."
I'd like to see this part re-written its been two days, I think its to early to kill the tension about zombies, keeping the reader puzzeled would be a better thing in this case.
You are still in the prolouge we are supposed to get to know the character here, its to early to give the main problem into the text. Which you kind of reveals in this part.
I skimmed though, but take this things into consideration.
Hope it helps!
-Massap2
We are the magnificent Masters, builders of pyramids.
