Precognition And Visions?

I've just gone up into high school this year (yes, I'm only 12, but I consider myself mature for my age), and a few things are starting to bother me.

Over recent years, I've started being able to 'know' things before they happen. Just little things, like what the next school play is going to be before they announce it, and that my drama teacher has dyed her hair before she walks into the classroom, and then I knew my best friend's grandpa was dead before my best friend even showed up at school. It was around then that I started getting worried.

Now, I've been getting these crazy dreams/daydreams. I guess you could call it that; I'm not sure what the word for it is. They happen quite often, but usually it's nothing serious, it's just something totally random, like I might see a particular scene, such as my history teacher yelling at someone for not doing their homework, which will then come true the next day. Sometimes, it's something that might have come true in the past, sometimes it's not so much a scene as more a still picture, like a girl in my class with a sprained ankle. Sometimes, the 'daydream' will only last for a split second, and so I won't really have caught on to what's going on in the picture.

I'm really worried now, because a few days ago, during a German lesson, I saw my dead 3-year-old cousin drown in a water tank. He died a few years ago, and I saw it all- I can even tell you what clothes he was wearing when it happened, the lay-out of the garden and even what the person who let him out into the garden looked like- the police never discovered who let him go into the garden, but I know.

I don't know what to make of this, only that it usually happens when I'm not concentrating on something- like I'm trying to get to sleep, or I'm waiting for the school bus to arrive, or I'm starting to tune out during a particularly boring lesson. I don't think I'm falling asleep; it's not like that.

How do I tell my friends and family about all this, without them thinking I'm insane, or that it's a joke? Am I actually insane? Please, help me.