Study the documentation, follow tutorials and practice... there's no secret.

You can read many topics already answered on the forum for this matter:

https://www.outsystems.com/forums/discussion/75095/advice-on-reactive-developer-exam/

https://www.outsystems.com/forums/discussion/77306/exam-questions-of-associative-reactive-web-needed/

https://www.outsystems.com/forums/discussion/66114/how-to-prepare-for-associate-reactive-certification-exam-in-outsystems/

https://www.outsystems.com/forums/discussion/72082/mock-exams-for-associative-reactive-developer-exam/