"I want to start a construction business. But how to start this i don’t know."
Forgive me but I cannot resist replying to this with the best advice you are ever going to get. One word. DON'T.
If you have no idea what you are doing, you are setting yourself up for disaster. I don't want to crush your dreams, but setting up a business of any sort, let alone something major like construction, is fraught with danger.
If you know nothing, then get out there and learn. Go and do courses to learn how to set up a business first. Get a job in a construction company and study every single aspect of how it is set up and how it runs.
My second piece of advice, write down every single thing you do from now on. If or when it all fails, you can earn millions by writing books and giving presentations to would be business owners telling them what NOT to do.
I have been on this forum for a long time now. There are hundreds of previous posts from people who have tried to start businesses and failed. Statistics show that all over the world, very few startups succeed. We all aspire to to Ambani's, Mittal's, Tata's, Musks, Zuckerbergs etc, but the world is not like that unfortunately.