That's a simple script. You can either use the script wizard or Lilac Souls script generator.

It will look something like this. Make sure you give the trigger a unique tag.

void main()
{
// Get the creature who triggered this event.
object oPC = GetEnteringObject();

// Only fire for (real) PCs.
if ( !GetIsPC(oPC) || GetIsDMPossessed(oPC) )
return;

// Only fire once.
//if ( GetLocalInt(GetModule(), "DO_ONCE__" + GetTag(OBJECT_SELF)) )
// return;
//SetLocalInt(GetModule(), "DO_ONCE__" + GetTag(OBJECT_SELF), TRUE);

// Set a local integer.
SetLocalInt(oPC, "nFoundBugbear", 1);
}

Place this in the trigger's OnEnter hook. If you want the module to restrict the script to only firing once uncomment the 3 lines under "Only fire once".

In future the best place for scripting help is here.