How can B.Tech Mechanical student get a job in PSUs?

I am a first year student of B.Tech.(Mechanical). What should I do get a job in PSUs' after my degree? Should I focus on theory or on numericals ? What will be better? There is almost negligible placement in my college. Is it will good to take admission in M.Tech.? But my priority is to get a job in PSUs'. Please suggest me.