I can feel what it can be like. I used to stammer when I was a kid.. a lot! Alhamdulillah I got rid of that on my own. You just need to understand why you do that. Mostly kids start stammering when they're under stress. It can be anything like peer pressure or something as simple as thinking that you would not be able to convey your message while speaking. These psychological reasons are very much similar to the ones causing other issues like that (bed wetting for instance) unless it is a physiological issue of course (like many people suffer head injuries and start stammering.)
If it is a psychological thing, let me assure you that you can get rid of it very easily. Like in my case, when I grew up a little, I tried to figure out why I did that (with the help of my parents and siblings of course.) Most people who stammer subconsciously fear rejection. They somehow believe that their words won't be heard and nobody will pay attention to what they say. In order to compensate for that, they subconsciously try to speak hastily (they just try to get it over with if you know what I mean.) By doing that, they refuse to acknowledge that their thought process fails to keep up with their words, so they stammer! You'll see that most of the people who stammer (due to psychological issues) try to speak hastily and in a hurried manner.
You just need to slow down buddy and realize that your words are precious! Have faith in yourself and have some self confidence. You're the best believe me! As Farrah said, you need to practice in front of a mirror as you utter your words. I mean talk to yourself.. I used to do that all the time when I had this problem. I also used to repeat the words that I was about to speak in my mind... slowly and calmly! For the time being, you just need a couple of good close friends you can confide in. They should make you feel comfortable with how you speak. They should remind you time and again to speak slowly and help you with your practicing. Wish you all the best regarding this my friend. You can always contact me through email and I'll be more than glad to help.

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...and I am the dark of this our new day