Learn the basics of the Internet

1. How does the internet work?
2. What are HTTP & HTTPS?
3. What is Domain Name?
4. What is IP Address?
5. DNS and how it works?
6. What is hosting?
7. What is SMTP?

Basics of front-end languages

1. HTML
2. CSS
3. JavaScript

Learn a back-end language

1. PHP
2. NodeJS
3. Ruby On Rails
4. Python
5. Go
5. C#

Just learn anyone of the above language but make sure you have an in-depth understanding of that language.

I will recommend NodeJs or PHP.

Learn Version Control System

1. Basic Git Commands
2. Repo hosting services
    I. GitHub
    II. Gitlab
    III. Bitbucket

Learn about Relational Databases

1. MySQL
2. PostgreSQL
3. MariaDB
4. MS SQL
5. Oracle

MySQL is the most popular one.

Learn about NoSQL databases

1. MongoDB
2. RethinkDB
3. CouchDB
4. DynamoDB

NoSQL is a very popular database. Many startups are opting for NoSQL databases instead of SQL databases.

Learn About APIs

1. REST
2. JSON APIs
3. HATOAS
4. Open API Spec and Swagger
5. Authentication
6. GraphQL

Learn about caching

1. CDN (Cloud Delivery Network)
2. Server-side caching
    I. Redis
    II. Memcached
3. Client-side caching

Web Servers

1. Nginx
2. Apache
3. Reverse Proxy

Understand web security

1. Hashing Algorithm
   I. MD5
   II. SHA Family
   III. Scrypt
   IV. Bcrypt
2. HTTPS
3. CORS
4. SSL/TLS

Learn testing

1. Integration Testing
2. Unit Testing
3. Functional Testing

Containerization / Virtualization

1. Docker
2. Kubernetes
3. rkt

Architectural Patterns

1. Monolithic
2. Microservices
3. Serverless
4. Scaling (Horizontal & Vertical)
5. Load Balancers