UPSC is a 3 stage   selection process involving Prelims, Mains and Interview which tests your personality test. Prelims has 2 Papers namely General Studies and CSAT.

General Studies has Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and Social Development, Environmental Ecology, Biodiversity and Climate Change and General Science.

CSAT has Comprehension, Interpersonal skills including communication skills, Logical Reasoning and Analytical ability, Decision making and problem solving, General Mental Ability, Basic numeracy and Data Interpretation.

You will need atleast to start preparing ahead of 1- 2 years. If you are preparing at home, you need to keep yourself away from distractions and focus on the goal ahead. Gather as much as information as you can on each topic and keep yourself updated. Practise mock test papers. The thing with General Knowledge is that it new editions keep releasing every year. So if you are in 11th right now, sticking to Newspapers might be a nice idea. Alternatively, to keep yourself updated to about the happenings of the world, Arihant Publications might help you. Concentrate more on History and Geography because they do not change as they have already happened. That way you are preparing from quite early on.