You can go through the steps as mentioned below in order to start your preparation for UPSC in a gradual manner:
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 integrated 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.
Start with basics, for each subject, you can do with NCERTs for this.
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.