Your absolute first step is to gauge interest.

Without demand, you will end up wasting your time and resources building something nobody wants.

Find a need, know it inside and out, then build the solution to that need.

Don't just talk to you friends and family (they are biased). Talk to the real users of your proposed idea. If you're targeting freelancers, talk to freelancers. As many of them as you can. Learn what they wish was different about XYZ, understand where the pain points are in current marketplaces, and find the weaknesses of the existing competition.

All the best,
-Shaun