First thing first. The full form of UPSC is Union Public Service Commission and it is a Constitutional body with various well-defined functions. One of these functions is to act as a recruiting agency in respect of top-level Central Government posts. The most well-known examinations conducted by UPSC are Indian Civil Services Examination, Indian Forest Service Examination, Indian Economic Service Examination, Indian Statistical Service Examination, Geologists' Examination, Combined Defence Services Examination and National Defence Academy Examination. The minimum academic qualifications and other eligibility conditions for all these examinations are different. In addition, UPSC also conducts various isolated Class-I posts recruitment examinations depending upon creation of vacancies.
So, you first learn about the eligibility of the aforestated examinations, chose which one you want to attempt and then prepare with all seriousness. Remember that these are all India examinations and best students appear in these examinations. So, start your preparation as early as possible.
"If you are killed in action, you go to Heaven. If you win, you rule this Earth (as beautiful as Heaven). That is why, O son of Kunti, take a firm resolve and fight!"-- Shrimad Bhagwad Gita