You know CSS and HTML which is quite good. Apart from this the skills you should learn are:

• JavaScript
• Back-End Languages such as-

1. PHP
2. Python
3. Ruby
4. Java

• Web architecture
• HTTP and REST
• Data Base storage (you need to learn how to work with XML and JSON)
• You need to know about the basic designing principles including including UI & UX design, prototypes, scalability, etc.
• You need to know about the NPM package manager.

To know more about how to Become a Full Stack Web Developer, visit the link below:

https://www.careers360.com/courses-certifications/hyperiondev-become-full-stack-web-developer-course

I hope this answer helps. All the very best for your future endeavors!