First time, but not the last.
My twin sister and I have always been close as most twins are. When we were young, our mom would give us a bath together. It wasn't out of the normal, and never looked at as sexual in anyway. We would look at each others 'Private' areas, and think that anything other that what we had, looked strange.
Then puberty hit! My sister hit puberty before me as most girls do before boys. I only knew that because her boobs had started to grow, and there were now pads kept in the bathroom that connected our rooms. At this point, the door was being shut when one another was using the bathroom. I can't really remember when I started getting 'morning wood', it had always seemed like I had them every morning, but it was just another part of my body. But before puberty had really hit, I noticed my boners feeling better. I became in love with my own dick. I would stare at it for hours. In bed, in the bathroom in front of the mirror, I just loved to look at it, I didn't know what else I could do with it alone. It still hadn't grown, or had any pubic hair, but it would feel all tingly. In the showers after gym class, I would pretend not to look, but in all reality, I would check out other guys dicks to compare to my own. Some had hair, some did not. The ones with hair would be bigger, with their balls hanging lower. There were more of us that didn't have hair yet, than those with. Only a couple of guys were early bloomers and every now and then, someone would have a boner. I would have to fight hard to keep from getting a boner. It took a while, but I admit now that I am bi. But that is a different story.
Anyway, back to my sister and I. One night, our mom went out when we were 13, which is still pre-puberty for me. Mom had been leaving us by ourselves without a babysitter for close to six months at this point. This night, she said she was going to be a little later than normal. Saying she would be back at 1am, she kissed us good-bye, and was gone.
We sat around watching TV, and played some PS3 games. It was like any other night until about 10 pm, we had our jammies on already. Hers was a tank top and shorts, and mine was a t-shirt and boxers. I had the remote so I was channel surfing and came across a soft porn on Cinemax. I had never seen anything like this. I hadn't been up this late in our living room watching TV before, and if I had, my mom was watching what she wanted to. As soon as I went to Cinemax, right away, there was a guy shown shirtless from the waist up and had a look on his face like he was either in pain or enjoyment, and then a woman popped up in front of him with her large tits swinging freely. I had the most incredible boner immediately. My sister didn't say anything, but my jaw hit the floor. I had seen pictures of naked women before, but this was my first time seeing anything with live action. Still amazed I didn't hear her say 'Turn it', I looked over at her (She was sitting next to me on the couch with about two feet in between us) She had her right leg over her left and kind of shaking her foot, and her arms crossed over her chest. She had a small grin on her face, and this is when I realized she liked what she saw and would have no objections to watching it. So I said...'You like this?' She replied 'It's ok.' 'Have you seen this stuff before?' I said, and she then goes 'Yeah, but this isn't like a real porn.' I was totally amazed that my sister knew what this was and more so that she liked it. She goes 'I will be right back.'
She came back into the room, and said 'I found this in Moms room a while back.' It was a DVD that she put in the DVD player. It was a hard core porn movie! I had put a pillow over my lap to conceal my boner, otherwise it would have popped right out of the open fly. What I saw was amazing! It was showing everything, Two women together was the first scene, and the next was a guy and a woman doing everything. This guy had a huge dick which made me feel a little self conscious about mine being so small. Then came a scene where the same couple were laying next to each other masturbating. I really didn't know about all of this, but it looked like they were enjoying it. My twin then asked me 'Do you do that....jack off?' I said no. I had heard that 'white stuff' comes out of your dick, but that was it.
So then she said 'Can I see yours?' I was a little hesitant, but in order to sweeten the deal, she said she would show me hers. At that point, she lifted her shirt and I saw her beautiful small a cup boobs, with very tiny pink nipples that were standing straight out, about the size of pencil erasers. My boxers were getting wet, but at this point in my life, I had no idea what precum was. So told me that if I showed her my dick now, she would show me her pussy. So I moved the pillow and took my boxers down. My hairless dick popped right out standing straight up. It was only about three inches long and about as skinny as my pinky finger. She then pulled her shorts and panties down in one swipe. From where I sat, all I could see was a small puff of blonde hair. This was all I could take. I had a spontaneous ejaculation. There was a lot of it, but at this point in my life, it wasn't white, it was clear. It wasn't precum, it was cum from my underdeveloped body. It shot up to my chest and stomach. Her eyes got very big, and then leaned over to get a closer look at it. My boner had no chance of going away. I was so incredibly horny still. She started to rub her pussy very slow. She stopped and reached over with her right hand and put it on my dick and pulled it up and down. 'Do that to yourself' she said. I took my index finger and thumb and rubbed my dick up and down slowly at first. She started speeding up her rubbing while staring at me stroking my dick. Her legs started kicking, her head went back and she let out a deep moan. She stopped for about a minute or so, then started rubbing again. About then is when I had my second orgasm, but my first one I ever made myself, just like the first one in volume. I was covered in cum, with some of it running down the side of my body onto the couch. I came one more time that night and she had two more orgasms.
I was thankful she taught me how. We have done this more since then, and done more too.