The first step is to clarify exactly what you are interested in. That is, precisely delineate your question. It is likely that you have been making observations, or perhaps one notable observation, that makes you curious. There is something you have seen that you are wondering about.
Now to engage in scientific enquiry, you must form the question deriving from your observations. Why do cats have stripes? would be far too wide to start engaging in any serious enquiry and you would soon allow that many cats do not have stripes, so you must refine your enquiry.