If you want to be a tech lead and not someone's job all you have to do is go out of your way to own the things you are directly working on and go out of your way to help those who are working with you to achieve shared common goals. Once you have done this multiple times, its easy to schedule a meeting with your management and pitch the idea of you doing the same and more but this time with Tech Lead's title and if that doesn't happen go look out for opportunities outside your company.