Note: This post considers that you have some background knowledge about web development ecosystem, also some common terminologies used in the web community
First, set up a strong git process. Two fixed branches master(prod) and develop and other closable features and release branches. You can refer for a more detailed explanation in this link.
Nicely broken down tasks according to the business requirement and priority and use a platform like JIRA/Trello/Asana to manage it.
Building and maintaining proper design templates using tools like InVision/Zeplin/Figma for UI standards.
One-step build, test and deploy probably using Jenkins. Refer Point 2 in Joel Test
Tests should include unit tests, integration tests, and end-to-end tests. Possibly automate testing cross-browser. Make sure your build fails if any important tests fail. Also, do take care of implementation details while testing your code refer to this link for more.
The codebase should be properly formated which can be done through Linters using recommended linting rules like Airbnb or Prettier.
Setting up web servers, if its an SPA, set it up using s3. If not then set it up behind a load balancer.
Setup error reports using tools like Sentry to capture errors happening at the user's end. Setup notifiers on email/slack to notify if something went wrong.
Use tracking tools to track user activity. Later can be used to study user behavior and maybe change UX accordingly. Examples: Google Analytics, Clevertap, etc.
Automate using the Lighthouse tool to get scores on app performance and study the impact of new features on those scores each time a production deploy happens. More info
Start with code splitting from the word go to avoid any hassle later. More info
To avoid Dev, Staging and Prod environment mismatch and all the problems that come with it, use Docker and club it with better tools like AWS ECS to manage docker clusters. More Info
I may have missed something that you may have learned through experience and I would like you to put it down in the comments down below. Do leave compliments too if you liked it. :)