You can go through the list mentioned in the link, https://www.indiastudychannel.com/forum/173507-suggest-some-good-sites-to-study-for-upsc-exam.
Go to all the websites and choose two or three best websites that you like.
As a former aspirant, my suggestion to you is to prepare from a Mains perspective, then you will automatically clear the Prelims exam. Choose the optional subject after thorough consideration so that you don't need to change the optional subject later.
Go to insightsonindia.com, where they will explain about how to write answers. After your preparations are over, you can join online Test Series, provided by some of the institutes like ForumIAS or VisionIAS. They will analyse your content and the writing style and suggest some improvement.
2. www.mea.gov.in and www.mha.nic.in. These two are government official websites.
3. .www.rbi.org.in. This website is the official website of the Reserve Bank of India.
There are many on their official websites of various ministries which will be good for various updates and reports which will be good for the preparation for IAS. The following are also good. But you can select the best sites based on the subject you select.
Thoughts exchanged is knowledge gained.
1. Study IQ - This channel gives complete information on the subject along with the current and also explains what kind of preparation should be done.
2. Dhyeya IAS - Through this website gives you overall knowledge for good notes and national issues.
3. Kautilya Akademi Youtube Channel - This is also a very good channel, from here you can also find the objective.
4. Nirman IAS - is a well-known organization, whose sites and YouTube channels will help you in preparation.