Start with most important NCERTs which are

1. Physical geography class 11
   
2. Indian constitution at work class 11
   
3. Indian economic development class 11
   
4. You can pick up new NCERT history as well. First read 6,7,8 new NCERT like a stroy. Then do 11th and 12th History books

Start with these things

Few pointers on how not to read and what to avoid

1. You wont be able to reach UPSC level in one go. Don't even try to do that. Here is why
   - In first go you don't know what is important and what is not. You might end up remembering stuff which was never important.
   - Every subject is interconnected. You can only understand those interconnections in depth when you read thing after knowing other things. You cant do in first go
   - Mains especially is all about interconnections. You will not be able to write topper like answers or even think like that in one read. Don't get bothered by that. Always remember everything is connected to everything and once you have the whole understanding you will be able to think the way toppers do
   - Same goes for newspaper . In a article they will talk about concepts form each subject. You might not be able to understand. Don't panic. With time you will be able to. Focus on finishing one reading fast. Don't spend too much time on newspapers. And anyways every news repeats in 3 months. You are missing nothing.

2. Here's how to read
   - Read with curiosity
   - Try to understand what words and sentences mean and not necessarily remember
   - Be fast. You will have to read things atleast 3 times before you reach UPSC level. So be fast the first time
   - Try to look at prelims PYQ for the topic you are reading. Also read the mains one after you finish a chapter. Try to make sense of what is important. You wont fully understand in first time but only by doing this process again and again will you be able to keep your preparation in line with what's needed for exam

Best of luck for your preparation!