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,
Hi, what kind of marketplace do you want to create? That would help us give you a better answer.
If you're selling products for the first time, you probably want to try eBay and/or Amazon before going into the time, money, and effort to create an online marketplace.
If you're selling services for the first time, you probably want to try Upwork, Guru, PeoplePerHour, Clarity, etc. before starting your own marketplace.
Also, a lot of experts are having success creating online courses on Udemy — but it's an art and science. I'm working on this myself and with my clients.
I would love to talk with you for even 10-15 minutes to help you get clear on your goals. I've been helping people like you for over 20 years and they get results — but we would need to target your specific situation.
Best wishes and I hope you book a call for personalized advice soon!