Operating Systems

You can pick an OS between Windows or Ubuntu. As majority of Indian colleges operate on Windows, there are few colleges which operates on Ubuntu. I haven't seen any college operating on Mac. So, it is better to stick with the technology used in your college. If you want to learn programming languages which you will be studying in your BCA course then you can check out this app "BCA - Course Programming" which is specially designed and developed for BCA students and is available on Google Play Store. It has programs and assignments of languages like C, C+ and Java with their outputs. You'll be studying these languages in your 3 years of BCA.
https://play.google.com/store/apps/details?id=technark.bca_courprogramming

Hello, You should be familiar with concepts of networking using Cisco routers and switches as well as subnets, and VLANs. The concepts are widely used through out the CCNA Sec course. The exam itself requires CCENT pass at the minimum to appear for CCNA Security exam. Other than this, there are no specific requirements for attending the CCNA Security exam. However, it is recommended to do CCNA R&S before preparing for CCNA Security as it would help you prepare well and also useful in career development. You may use official study guide along with practice tests for preparing for the exam. CCNA Sec is considered to be a bit difficult to pass and thorough preparation is required in similar lines with CCNA R&S. If you are already working in related area, you can take the exam directly after completing CCENT (a pre-requisite at the minimum). Good luck!