Hello,

Right now you're only in 11th standard and you've got ample amount of time to prepare and appear for UPSC, it's good that you've initiated towards the preparation process with NCERT books but make sure to focus more on your studies because that's a priority for you right now. Anyway, in case of UPSC it takes one year to prepare and another one to go through the process of examination. I feel the best time for you  will be during your graduation period.

UPSC Civil Service Exam consists of three level-

Preliminary
Mains
Interview


Preliminary numbers are not added in the final list, you just need to score the required cut offs against your category.It mainly consists of GS Paper-I and CSAT. Both consists of 200 marks, MCQs based.


Mains is more of a subjective type, one needs to write answer for Four General Studies Paper,Two Optionals, One Essay, two language paper, there are total nine papers here. 250 marks for each paper, total is 1750


As I mentioned the biggest advantage you have right now is time and if you use it wisely, it'll help you succeed in the time to come. You can go through the gradual steps as mentioned below for preparation of UPSC during your graduation period:


Be well acquainted with UPSC Syllabus, learn it by heart, that's the most crucial step to crack this exam, to know what to read and what to skip. In addition to this, never ever make a differentiation between prelims and mains, what I mean is do not prepare separately for each, because nowadays what's happening is even in prelims, UPSC frame questions based on Mains syllabus, So, the first towards preparation shall be a combined preparation for both.


Read any newspaper like The Hindu, or Indian Express on a regular basis to be updated as well as gaining knowledge which shall be highly useful to you in future exam preparation part.

After going through NCERT books as you progress, move on to other relevant books, keep your resources minimum like Polity by Laxmikanth, History of Modern India by Bipin Chandra etc.

Start practicing answer writing in advance, even in a little way, writing mains is a huge task for nine paper, it requires immense practice along with knowledge.

Practice previous year question paper which you can get from the site of UPSC,in addition mock papers, and test series which you can get it from any of the sites extensively dedicated to UPSC, , it's highly important to evaluate your performance while doing so.


Prelims is mcq based, but still it requires knowledge coupled with speed, and for mains, as I already mentioned the importance of answer writing.