You can appear for CAT whenever you want to. Also their is no restriction on age and number of attempts for the examination. You can start your preparations as early as possible by utilizing whatever free time you get after your B Com studies.

For preparation you can refer to following reference books as per your choice: 1) How to prepare for Quantitative Aptitude for the CAT How to prepare for Data Interpretation for the CAT How to prepare for Logical Reasoning for the CAT How to prepare for Verbal Ability and Reading Comprehension for the CAT 2) Pearson Guide to Quantitative Aptitude for CAT by Nishit Sinha Pearson Guide to Quantitative Aptitude and Data Interpretation for CAT by Nishit Sinha Pearson Guide to Verbal Ability and Logical Reasoning for CAT by Nishit Sinha There are others books like Word Power Made Easy by Norman Lewis, High School English Grammar and Composition by Wren and Martin etc to help you for your preparation. You can also join various coaching institutes like TIME, CL, IMS etc or the online coaching institutes also for concepts and mock tests.

I hope my answer helps. All the very best for your future endeavors!