1. The best tech stack to start learning backend would be Node JS & Express JS framework. There’s enough learning content available on YouTube. Start building with a simple hello world project and slowly increase its complexity.  
 Some good projects for hands on could be - building RESTful APIs for apps like Swiggy or even an app like grapevine. 
 2. You don’t need to learn frontend to validate your backend code. But you must identify your use cases such that that they are based on CRUD operations. These APIs can be validated by using tools like Postman.

Software Engineer

Software architect

I'm planning to learn backend.
1. After backend basics, which language and framework should I start with
2. Should I have to learn front end technologies too?

I'm planning to learn backend.
1. After backend basics, which language and framework should I start with
2. Should I have to learn front end technologies too? 

From google getting very generic response.

Blunt advice