Here goes my answers to your queries:
I failed in class 12 .Now what should I do?
Prepare and give it again. It will be second attempt but you have to score beyond passing.
I am very confused? I want to go in technical line.
Technical line is good and booming but it requires some interest in Sciences and Maths. Are you interested in these subjects? Do you find them easy to study and comfortable? It would be a disaster to opt for some line just because others are doing so. Find your interest and passion. If it is technical, then go ahead and take a technical line.
Should I opt for BTech after clearing 12 or after doing polytechnic.
Check admission eligibility criteria for both BTech and polytechnic. Both need passing and beyond %. So in the current state you are not eligible at least for Btech.
What is your SSC %?
If you are eligible for polytechnic, then opt for it.
If I do polytechnic which line will be best: mechanical, computer science or electrical engineering?
Instead of asking experts here regarding which line will be best, ask yourself: what interests me? Are you interested in coding or programming? Do you have a liking for electrical circuits and wiring or for machines and gadgets? Based on what interests you , choose either of mechanical, computer or electrical program. do not base your decision on heresay. The choice will decide your future. If it is interesting for you, then you will enjoy being in that field for lifetime.
If I get degree after diploma, will it have the same value as compared to those who get degree after 12?
You are going to get the same degree, only the entry level is different. So no worries, the value of the degree is same. After you complete your degree, no one is going to ask you whether it was after 12 or after diploma. Your scores will say it all.
Hope this answers all your queries.
Autograph your work with excellence!