Build up the Basics:

Before actually starting your preparation, go thoroughly through the syllabus and exam pattern once. The syllabus is divided in three sections:

Section 1. Verbal Ability and Reading Comprehension

Section 2. Data Interpretation and Logical Reasoning

Section 3. Quantitative Ability

Building basics is the most important part of your preparation. Only after understanding the basics you can solve all the problems. For this, you can look at online courses or classroom courses or you can prepare by yourself. I would go with online courses as these save the time of traveling, 24*7 hours available and are affordable. As you are writing CAT 2019, I am assuming that you are in your college and it would not be easy for you to find time for going to class. So you can take help of online courses. 

Practice:

After understanding the basic concepts of a topic, get a book from market and start practicing questions from it after going through the concepts for that topic. If you stuck while doing the questions, go through the basics again and then try that question. Try out previous years questions as well. Although previous years questions from CAT 2009 are not available in public domain as the exam went online from this year, you can practice the questions till CAT 2008.

Analysis: Analysis of your performance is as important as practicing questions after going through concepts. After completing 70–80% syllabus, you can start writing mock tests. You’ll know your weak and strong areas by attempting mocks. Once you know your weak areas your next task is to work upon them to convert them into your strong areas. Practice more questions until you excel in those topics. Write at least one mock test every week.

All the best!