This story is the first time I ever encountered a ghost. Lucky for me, it wasn't in my house, but it was still enough to scare me for quite a while. I was about 11 or 12 at the time.
Growing up, I believed in ghosts like you would believe in Santa Claus or the tooth fairy-- it was something I thought was real but had no evidence to back it up. Then, when I stopped believing in things like Santa Claus, I stopped believing in ghosts as well. It was in this doubting period that I saw my first apparition that caused me to believe in the paranormal again.
My friend Natalia, who was 14 or 15 back then, told me that when she and one of her other friends had visited her grandpa's house that week, they had seen the full-bodied apparition of her dead grandmother. I got angry with her and told her how crazy she sounded. She insisted it was the truth, and invited me to spend the night with her at her grandpa's house to prove it. I decided to take her up on that offer to validate my claim that there really was nothing there.
We went over to her grandpa's house and set up somewhat of a stakeout in the room she saw the ghost. There was no professional equipment involved, just the two of us talking and keeping our eyes open for anything out of the ordinary. At around 12, Natalia went to get snacks from the kitchen and left me on watch by myself.
A few seconds after she'd left, an old lady walked into the room by a different entrance than Natalia had used. She stared at me kindly as I stared back, my eyes locked on hers. Just before Natalia walked back into the room, she broke eye contact and exited the same way she had came.
When Natalia came back, I asked her if her grandpa had a girlfriend or a friend that was in the house. When she said no, I told her about the lady I saw and what she looked like. She showed me a picture that looked exactly like the person I had seen. Her eyes got wide and she said I had just saw her dead grandmother.
I was shocked, but looking back it became more obvious that the woman did seem a little odd. For starters, the way she just walked into the room with no intention other than to stare at me was really out of place. Then, as I thought some more, I realized she didn't have a noticeable shadow -- and the way the lights were shining she definitely should have.
The rest of the night went on without incident. What happened made me a believer, and I've never doubted the existence of the paranormal after that, though the experience frightened me so much I decided to leave ghosts and everything related alone for a couple years.
When I asked her about it recently, Natalia said that since that night her grandmother has never been seen again.