My first exposure to masturbation was watching my brother enjoy himself.
When I was a kid I shared a large upstairs bedroom with my brother. We had separate beds. He was about 3 years older than I was, and neither of us made much effort to hide ourselves while dressing, showering, etc. So I was used to seeing him naked, and it was no big deal.
Sometime around when I was 9 and he was 12, I noticed him moving around in bed. I usually fell asleep quickly, so he might have been doing it before, but this one night he caught my attention. I looked over, and with enough light coming in the window to see him clearly, saw that he had pulled his boxers down and was laying on top of his pillow, rubbing on it. I was curious, and watched. After a while he sped up, then looked real tense, and stopped, just moving his hips slowly. I had no idea what he was doing, but was surprised to find my little penis hard, since I figured it had something to do with making his penis feel good. After he lay there like that for a minute, he rolled over, and I could see that his penis was hard, and there was a little white, stickly looking stuff on it, and I could see a little wet spot on his pillow. He paid no attention to me watching him as he wiped up the mess with a corner of his sheet, pulled his boxers back up, and settled down to go to sleep.
I watched this little show almost every night for a while, and finally asked him what he was doing. He showed no sign of embarrassment, and just said it felt good and I'd find out in time.
I don't know how long it was after that that I started experimenting myself. I always got hard while watching my brother, and one day I put my hand down in my pants and humped my hard penis on my hand, and it felt really good. So I slowly pulled my pillow down to my crotch and rubbed on that, and that felt really good, too, so finally pulled my shorts down and got on top. It took a little while to get it right, but pretty soon I was humping a crack in the pillow just like my brother, and without warning, I had my first orgasm. I remember that it just kind of snuck up on me, and all of a sudden my penis was throbbing and my whole body felt so wonderful. I gasped and grunted loudly, and my brother heard me and looked over. He had just had his orgasm, and hadn't noticed what I had been doing, but he could tell I had just had an orgasm, and said, 'See, doesn't that feel good?' I was in too much ecstasy to answer, as my orgasm was still in progress, so I just grunted in reply and kept holding my penis pressed into the pillow.
As I came down off my first orgasmic high, my penis got soft again and I rolled off my pillow, exhausted but with a great feeling of relief and well being, and tired beyond belief.
Of course that became a normal evening routine, with my brother and I humping our pillows side by side every night. After a year or so, my brother started using his hand, but I kept using my pillow.
I remember one night when my brother had a friend over, and they slept in his bed. It was pretty tight, but they didn't seem to mind. I figured our nightly activity was off for the evening, but was surprised when they whispered for a minute, then both pulled their shorts down and started masturbating side by side. I watched in fascination, and decided what the heck, I'd do it too, and pulled down my shorts and mounted my pillow. I noticed that his friend was watching me -- I think he was surprised that a boy so young (I was maybe 10 or 11 by this time) did it, too, and he seemed very interested in seeing me like that. I didn't mind, and in fact it felt exciting to be watched. He wasn't the only one of my brother's friends that I put on a show for; they all seemed to enjoy watching me.
All this went on until my brother moved away to college, but we'd do it together when he was home. It just seemed like the natural thing to do.