CAT and CLAT are completely different. For appearing in CLAT which is a law entrance exam you'll have to have completed 10+2 but for CAT which is a management entrance exam you'll have to be a graduate. The syllabus for these two exams are also very different, only a few things you can get in common. But you'll have to choose either of them in order to proceed.

Regarding preparation you can follow different books of quantitative ability and Logical Reasoning, English grammar and law for CLAT. And for CAT just practice the previous year questions, some quantitative ability concepts and read editorials.

And try to get test series for both the exams.

All the best