Scientific method requires a good hypothesis, a question or an explanation of why something happens or what will happen. Experiments are then conducted and data is collected and analyzed to test the hypothesis. Based on the experimental results, the hypothesis is verified or modified or rejected. 

Therefore, a good way to come up with a scientific question for an experiment is to hypothesize, that is, coming up with a hypothesis. A good and valid hypothesis is testable and fallible. This means that we should be able to test it using experimentation and if need be, we should be able to reject it or modify it. A hypothesis is not just any random guess, it is an educated guess based on available information. A good question or a good hypothesis will be based on what the experiment hopes to measures and the reason behind it. 

Hope this helps.