Have you recently been having a bad mood? Any bad incident in particular? Even tiny incidents can cause stutter. This is just a guess, but could it be because of puberty? You have just started undergoing puberty and changes in the vocal cord could be playing a role. Also try thinking about the possible changes. It may take some time. It could be temporary, who knows. Ask your family about it. Remember that this is temporary, stutters have been observed to go as fast as they come. In the mean time, focus on A) Breathing to calm yourself down. B) Building your confidence by actively taking part in social interaction. Exercising is another great way. C) Speak slowly. Focus on those syllables and letters you stutter and try to extend them like - "Apple" becomes "Aaaaapple" D) Try to observe your mouth/throat movement. Many times, you will find it that it's uneccessarily strained. Work on the stutter NOW. Else it ends up being a habit which makes it tougher to get rid of.