Sit

How do i make a script detect someone standing up from sitting on the object the script is in?

The changed event. The example code has almost exactly what you are looking for, except backwards.

i know, i tryed and coulnd get it to work

I dropped a copy on you in-game, but just so everyone can play:

// almost completely stolen from the LSL Wiki

string name;

default {
    state_entry() {
        llSitTarget(<0, 0, 0.1>, ZERO_ROTATION); // needed for llAvatarOnSitTarget to work
    }
    
    changed(integer change) { // something changed
        if (change & CHANGED_LINK) { // and it was a link change
            if (llAvatarOnSitTarget() == NULL_KEY) { // somebody is no longer sitting on me
                llSay(0, $name + " stood up");
            } else {
                name = llKey2Name(llAvatarOnSitTarget());
                llSay(0, $name + " sat");
            }
        }
    }
}