Advice for new data scientist at mobile app startup

I just joined a friend's mobile app startup as their first data scientist, and I'm pretty new to the field- being new we know I won't do the best job, but I'm eager to learn.

I want to get started as painlessly as possible.

What are the best resources for orienting myself, understanding which metrics to focus on etc

What should I use for the data stack? I want to lay good foundations without overengineering. We have data from Apple App Store, Cloudinary, OneSignal, server backend, and ads on Facebook and TikTok.

Any other pieces of advice?