imo the best way is to look at what other's have built. Here's some of my favorite talks that go from 0 users to millions.

Dropbox - https://www.youtube.com/watch?v=PE4gwstWhmc

Instagram - https://www.youtube.com/watch?v=oNA2C1vC8FQ

Slack (bonus. not as applicable but good reminder of why initial architecture does matter) - https://www.youtube.com/watch?v=WE9c9AZe-DY