How to Insert data one table to another table

Question

sir,

I am creating a website where i am add a login page so where i want when the user is login then that user's user id is auto matick insert where registred user's user id when the user is upload any data in my site.
ex. database:- register_tbl
      where column: user_id
  user_name
  mobile_no
 data table2:-  complain_tbl
      column name: user_id(copy from register_tbl)
         complain type
   complain
i want this type of connection to copy or retrieve data from register_tbl to complain_tbl where user_id.

pls help me.

and see the this picture what i want.