Good ISTQB preparation is not complicated, but it does need structure. Start with the official syllabus, use practice material that matches the exam style you will sit, and make sure you understand the terms the way ISTQB defines them. If you want the chapter-by-chapter version, go straight to How to Pass ISTQB Foundation Level. If you want the clean starting point, use the resources below.
A simple way to prepare well
Read the official syllabus once to understand the structure. Read it again with notes. Work through sample questions after you finish each section. In the final stretch, switch to timed practice so you get used to how the exam feels under pressure.
If you are preparing for Foundation Level, the strongest next pages are How Hard Is ISTQB Foundation Level?, ISTQB Practice Questions, and ISTQB Online Exam.