I chose the overall system architecture and started working on the API.
At this time I knew I will be using Node.js + MongoDB for Backend and React for Frontend.
When you start working on something remember to not overthink the architecture. One of the key rules of a good software architect is to make your decisions as late as possible. This will give you the flexibility you will need.
For technologies - go with what you already know and feel comfortable with. You will be spending a lot of time, solving problems that you don't expect.