i have 70% in b.tech mech.engg.and 87% in 10th and 67% in 12th.so how i shld prepare fr psu?? and on wht subjcts i shld focus???

The exams organised by not and ongc will cover all the mechanical engineering subjects with some weight age. If you want in specific for about two PSU's, you need to work more on thermal subjects (Thermodynamics, Thermal Engineering, powerplant engineering and Heat transfer). But you should also be good in other core subjects also because that marks also necessary for clearing the exam.
For better understanding please go through last two years papers.
Fluid mechanics, Strength of material, theory of machines , production technology also need to learnt.
Note:- Fluid mechanics, strength of materials and thermal subjects are core things for major exams.
All the best.

Dear Abhinash
You got good percentage in all. But for PSU's it's depends on different one's, because psu is not like intermediate it's the collection of so many fields, where so many jobs are there. Please do smart and prepare accordingly.
All the best for your bright future.