Which mechanical related business can I start?

Santosh, welcome to CE. While you should start business depending upon your interest and area of expertise, the truth is that you need to start a business to fulfil an industry need. 

This need could be a problem that industry is facing with or something that can be done better to save costs and time. In short, you need to start a business that solves some kind of problem and that people are willing to pay for.

Instead of suggesting random ideas (which you can any way discover on the Internet), my suggestion would be to 

1. Find your area of interest and expertise
2. Find out how those skills can be used to fix a problem or do something in better way than it's done currently
3. Try creating a solution in the form of product or a service and take it to your potential customer. See if you can approach 10 potential customers and have at least 2 buy your solution.

I'm sure you are capable of coming up with ideas on your own that you can work on. I hope this helps. Feel free to ask any questions you may have.