my 9months old baby is still not able to sit unassisted nor able to stand. wondering is this normal? do I have to teach him to sit or stand? or they will learn by themselves?

From your observation, is there anything you think is not right, eg his neck muscle seems weak or he gains only little weight ? By age he already supposed to be able to sit on his own, though every child differs. Still, follow your instincts and ask your paed to do assessments,coz at least if there's something need attention, it can be done early.