Lock Object in Module Pool

Hi Experts,

I have created two screens 100 and 200.

In 100,i have button 'CHANGE'.When  i click on change button  called 200 screen.

My question is ,when multiple user using same Z-Transaction ,first user doing changes in 200 screen so second user need to call 200 screen in display mode when first user making changes.

How to do this?

i had created lock object and called FM in my program but i am not getting it.

Please give me any solution.

Thanks and Regards,

Ravi Teja