Hello krishna,
If you are failed in 12th, then how to take admission in engineering..you can not join b.tech.
But there are another way
You may try for a diploma in engineering. This is also called as polytechnic. The basic eligibility is class 10th. So, even if you fail to clear your 12th, you will be able to take admission. But, there will be a gap of 2 years in your education (may be more than 2 years if you try to clear 12th several times).

After having your diploma done, you may try for a B.Tech/B.E degree from an institution through lateral entry. You will directly get admission in 2nd/3rd year of B.Tech (depending on the university rule

Hope this helpful for you.
Thank you.