Class 12-Commerce

Accounts and mathematics: Start practicing questions on all the topics. Get all the previous year board papers, and solve them as if you are giving your final exam. Don't forget to learn the theory of accounts, it's equally important.
Business: Understand each topic and try relating it to the real business environment (or as if you are opening your own firm, what all points you would consider) and you will be able to tell the actual advantages and disadvantages on your own. Just a little bit understanding is required.
Economics: Understand the theory, practice the graphs, numerical.
English: You can't do much for the reading part. You can practice previous years writing section. For literature devote one full month to NCERT text books and read the chapters again, so that you can actually write the answers in NCERT language form (because checkers search for those main words of text book).

CCNA is a CISCO certification for Network administration. CISCO certifications are well recognized certifications which can help candidates to have long term well paid networking career. You may refer to following link for details of all CISCO certifications: http://www.cisco.com/c/en/us/training-events/training-certifications/certifications.html#~Cert. CISCO also has specialist certifications. It is recommended that you can start with CCENT and the CCT certifications to start career as a networking professional. I will suggest that if you want to have Network engineer career then do at least BCA degree otherwise you may face challenges as most of CCNA in India are B.Tech or BCA/MCA.