Prerequisite 1: Understand the basic concepts of unit and integration testing.
It's important to understand what software testing tries to achieve, and how unit and integration testing help toward that. Read about testing in theory (i.e. independent of any particular language) and understand things like black box vs white box, equivalence partitioning, validation vs verification, and the contrast between proving a program correct and empirically testing it. Any good software engineering book will discuss these.
Prerequisite 2: Be able to write basic test cases using frameworks like JUnit.
This is actually quite straightforward. If you can write some code, write a test class that tests at least one method, and run the test class so that it executes correctly, you meet this prerequisite. Most IDEs make this very simple by building JUnit into the IDE. Try it out in your favourite IDE.