Why would you need to kiss someone on the mouth when you are twelve? That's silly, but if it's what kids do these days..
You don't. You'll get cooties.
Be careful about the cooties. They are contagious and sometimes deadly.
I'am to 12, Even I am going to kiss my gf in this week. I say that, If she would kiss me on lips it will be great. Just go ahead be calm, look in his eyes for a while and then gently kiss him. He would be very Happy. Really !!! Tiffany I am older than her by 2 years, but its true love so it doest matter.
Depends on how old that boy is. If he is your age, then be calm and go with the flow. Don't let it get to sloppy, start off gently kissing until you get the feel of it. Most of the time kissing comes natural. If the boy is older then you by some years, don't even do it!