CAT is Common Admission Test for the management degree course in IIMs and other institutes.
UPSC( Union Public Service Commission ) is India's central agency and will conduct the Civil Services Examination, Indian Forest Service examination, Engineering Services Examination, Combined Defence Services Examination, National Defence Academy Examination, Naval Academy Examination, Combined Medical Services Examination, Special Class Railway Apprentice, Indian Economic Service/Indian Statistical Service Examination, Combined Geo-scientist and Geologist Examination, and Central Armed Police Forces(Assistant Commandant) Examination and others.
For both the above the important point is you should complete your graduation first. Without completing graduation you may not be able to do any one of the above two. So immediately you should concentrate on your graduation and complete that. Then based on your score and your interest you can choose any one of the above two. Both are tough and very hard work is the requirement.