How do you know when it's time to scale/optimize your tech and how to approach it?

I'm clueless about scaling, however my platform is growing and I'm concerned that my site is going to slow down. According to google analytics I had 400 people on my site at once today.

What are things I need to consider? Is there a way to know if I need to scale? How can I tell if my site has slowed down? How can I figure out what I need to scale? How do I figure out what the bottle necks may be.

I know thats a lot of questions, but really I'm just looking for any advice/resources for a beginner.

Tech:
frontend: Netlify, React
Image hosting: AWS
Database hosting: mongo atlas
Backend: Heroku, node/express

Resources

After I posted this questions I found these resources helpful:

1. Scaling an Application (video)

This video walks through a high level view of scaling an application overtime.

2. scaling on heroku (video)

Apparently you can do some scaling with Heroku really easily:

3. How I scaled a website to 10 million users (web-servers & databases, high load, and performance (video)

4. Web Server Bottlenecks And Performance Tuning

• start with optimizing the frontend

The video above mentioned threads vs processes. this video is a high level explanation of the difference this one does too