I got 80% marks in 12th.what is the best option at this stage for doing b tech in CS/IT/Mech if could not get good college under ipu.

If you don't get a good college under IPU then you can try for admission in private universities like Lovely Professional University, Manavrachna, Galgotia etc. Most of them offer direct admission on the basis of your marks in 12th. Check their websites for more details. Good luck.