How can I copy a standard table to make my own z_table.

Go to transaction SE11. Then there is one option to copy table. Press that button. Enter the name of the standard table and in the Target table enter Z table name and press enter.

WE CAN CREATE A STRUCTURE LIKE THE SAME STRUCTURE AS DATABASE TABLE AND WE CAN USE

SELECT* FROM DATABASE TABLE INTO TABLE ITAB

OR

INSERT INTO ITAB VALUES DATABASE TABLE

Use SE11 & there is copy(ctrl+f5) button is available after copy activate the Ztable, it's simple to copy a database table, but note that if in database table maintenance generator is available then you have to define your maintenance generator in your copied Ztable.