Yes, you can start preparing, but you can give the first exam or prelims only in the final year of Graduation or upon completing Graduation.

Also your age must be 21.

Hii aspirant..

For UPSC exams, the eligibility criteria is- 

The candidate should have done graduation from any recognized university. The candidate who has done professional and technical courses should also be from the recognized university. The candidate who is in the final year of graduation can also apply for the examination.

Thus after doing your graduation where eligible for the exam. Although for initial preparation I would recommend you to to read newspapers daily, get yourself used to with general knowledge topics and  try to Masters in any subject which you can opt for mains exam.

Other than yes I would recommend you to get involved in the preparation in the final year of graduation.

Hope it helps. 

Best of luck.