The best way to prepare is become aware and attain knowledge by reading more and more of current affairs. You can also get enrolled in top universities (like universities under DU) that offer the best courses in history and political sciene (if that is your field of interest). If you wish to join coaching institutes, you can apply for

Plutus IAS coaching (online)

Vajiram and Ravi IAS coaching (Delhi)

The Hinduzone (online)

Great that you know what you want to do right from class 12th. I think this is perfect time to start your preparation journey to be a part of steel frame of our country. Let me come to point on what and how you can start.

1. You need to be a graduate in order to be eligible for Civil Services exam.
2. So, you have 3+ years ahead of you to prepare yourself for the exam.
3. You have an advantage as you have 3 years to prepare yourself without taking much pressure. After your graduation , you won’t need much time to prepare.
4. First of all, choose your graduation stream very carefully so that you can prepare for UPSC alongside. Still , I would advice you to opt for a professional course instead of simple graduation as you gain a better perspective of things in a professional course.
5. If you can score top marks in 12th and can get admission in top 5 colleges of DU(or any top college), you can go for simple graduation too as your college add value to your personality.
6. Start reading newspaper. The Hindu, Indian Express, Hindu Business line etc. are some of the most followed newspapers by UPSC aspirants. Make this habit now to reap the benefits later.
7. Don’t just read the newspaper but make your opinion about the news you read. Think about the Why and How of things. This will make your task easier.
8. Read NCERTs and complete them during your graduation in order to get your fundamentals cleared.
9. Get your laptop and bookmark following websites : PIB , IDSA, Mrunal, Insights, IAS Parliament etc. Read the articles in your free time.
10. You are in no pressure to stretch yourself. So, you can prepare in fun way.
11. Make your notes and do revise sometimes. This will help you in real battle.
12. In your 3rd year, start thinking about the preparation seriously and target your first attempt right after graduation. Build your base during first 2 years. If your graduation is a 4 year course, you have one more year to capitalize.
13. Download the last notification, read it and save the syllabus. Go through the syllabus. Choose your optional based on your graduation subject preferably . If your graduation stream is same as your UPSC optional, you have dual advantage. Make strategy and targets.
14. Form a study group of like-minded people.

You can think of some other things which you can do to achieve your goal. You have time in your favors. Watch some videos by toppers and make your mind. Think again before embarking on UPSC journey as it is very tough, sometimes depressing and uncertain but once you decide, don’t look back. Those who keep trying will ultimately succeed. This is a game of will and discipline.

All the Best, Cheers!!!