Civil Servant (IAS/PCS)

There is no minimum percentage requirement. You should only have a degree certificate from a recognized institute.That's it. Generally the UPSC board doesn't give preference to academics. Its purely based on marks scored in UPSC exam.

Yes, after cracking UPSC prelims and mains with good marks, you are eligible for personnel interview. As per my knowledge, if you crack the interview, you will be an IAS officer. You have to study very hard for that. Your world should be limited up to you and your studies.
Best of luck.

A search on the net will lead you to many coaching institutes for IAS preparation. Getting a comparative study is also easy now-a-days. I want to tell you about some latest options for coaching. Many people are opting for online coaching classes which are affordable and good. They are conducted by experts so you need not worry about their effectiveness.
For example a visit to a site WizIQ.com will show that there are fifteen different types of courses (https://www.wiziq.com/courses/search.aspx?qry=IAS) available. You will get an idea about their popularity from the number of students taking the course. No harm in exploring.

After completing your class 12th from any stream, you can do your graduation. A civil service exam is conducted for which graduation is the minimum requirement. You can join a coaching to prepare for this exam.

To become an IAS officer, you need to qualify the civil services exam. For which you need to complete your graduation. IIT is an engineering institute, and to get admission in it, you need to appear for an entrance exam just after you complete class 12th (I don't know the aggregate). Both of them have a great scope. You can opt for any of them. You can appear for civil services exam after completing your B.Tech from IIT.

If you are sure that you want to be an IAS officer, you should go for BA in public policy & administration course in UPES, Dehradun. As the subjects taught in this course are of political interest and this college is also providing IAS training classes along with this course.
You can check more about this course on their website.

You can complete your 12th with atleast 60 % marks or more to get admission into good college for b.a. After graduation with 50%, you are eligible to take UPSC exam.
All the best