Scalability starts with clear boundaries. Separate presentation, business logic and persistence so each part can evolve independently. Define your API contracts and validate data at the edges.
Ten habits make a useful foundation: measure performance, define service boundaries, paginate large lists, index frequent queries, cache deliberately, move long tasks to a queue, validate inputs, automate critical tests, monitor errors and document operational procedures.
Avoid designing for hypothetical traffic before you understand real usage. Establish a baseline, find the bottleneck and improve the part of the system that is actually limiting your users.
