This story happened when I was about 13 years old. I had never done anything with a girl before--not even kissed. I had been masturbating for a few years, and I had a pretty good understanding of sex and sexual terminology.
I was at a small party. Everyone was downstairs but then almost everyone went upstairs to watch a movie or TV up there. There was a computer room downstairs, and only me and another girl were in it; everyone else was either across the hallway or already upstairs.
The girl, to be referred to as 'G,' had on a skirt and a shirt. The party was at her good friend's house, so she said she was going to grab another shirt to change into because she was hot in hers (she and her friend apparently do this all the time). She came back in the room a few minutes later, and I was still on the computer, but no one else was in the room besides the two of us. She said she had found a good shirt but also another skirt because the shirt didn't match her skirt.
'G' told me not to look because she was going to get changed in the same room. About 20 seconds later, she said she had a problem and she needed help. Her skirt was off, and her shirt was halfway off, but her bra had come undone in the back. So here she was, holding the front of her bra on with one hand and holding her shirt and the other clothes with her other hand. She asked me to fasten the bra for her, which I did.
I was expecting her to tell me to turn around again and go back to where the computer was, but she said something that completely surprised me: she said, 'That made me really horny, will you do it again?' She told me to unfasten her bra, which I did. I fastened the bra again, thinking that this is what she wanted me to do. I was completely excited, because once again, I had never done anything like this before in my life.
'G' then told me to stand facing her. She dropped her other clothes and was standing in front of me, wearing nothing but her bra and underwear. 'G' asked me if I had ever done something like this before, and I said no. Then she said, 'Well do you want to?' Not knowing exactly what she meant, though not caring very much, I said yes.
She unhooked her bra and took it off. She had big, perfect round breasts. 'G' told me to feel them, which I was about to do anyway. Then she took off my shirt and I took off my pants. I had a huge boner, as was evident from my boxers. She came over and pulled down my boxers, and at this point we were kneeling/sitting on the floor. I pulled down her underwear and exposed her pussy. We were both completely naked. She took my hand and moved it to her pussy, and sort of showed me where to touch her inside. She started stroking my penis. After a little of this she came right next to me and started to kiss me, her boobs rubbing up against me. We laid down on our backs next to each other, and we both masturbated. It was an event I will never forget.