What type of motion can I file for a child who is now 18 and decided to start living with me but is still going to school

I'm not sure what kind of motion you are referring to.  If the child is age 18 but is still going to school, you may be entitled to child support from the child's other parent.  If the child is no longer attending school, and you are paying support, you can seek to stop the support and have the child emancipated.