Hi Syed Fazle Rahman! I honestly don't do TDD if I'm not sure how I want things to look/behave. I'll often experiment a lot and throw away a lot of code before I get an idea of how I want things to look.
That said, if I do know ahead of time what I want an API to look like then TDD works great. But yeah, while tests generally save time in the long run, they can really slow you down if you don't know what the end result should look and behave like. Embrace the experimentation and test it when you like it.