Im in a placement and I would like to know that if I turn 18 can I sign myself out of this placement?

I don't understand what you mean by "placement." Are you in the custody of the legal system?

Hello a:

Like Judy, I don't know of any placements having to do with children and corporations...

But, if you mean a family placement, when you turn 18 you become an adult and may make YOUR OWN decisions about where you live.

excon

And then there's this: https://www.askmehelpdesk.com/parenting/when-you-turn-16-can-your-parents-still-tell-you-what-do-control-you-423546.html#post2122655.

Sounds like a problem where OP was put into some sort of supervised situation for not obeying "someone's" rules. Or not.