is it true that when newton was sitting under a tree, apple fell on him and this made him to realise force of gavity

No it is not true that apple fell on him and he realised the phenomenon of gravity.

 

It is however true that he was sitting in apple orchard and observed an apple falling and thought that apple's velocity increased from zero to some value when it hit the ground. So a force does act on it and he called it as gravity.