Think of a brief cohort of people that you would want to create a database for. Come up with the fields and data that you would need in order to get the best information for your study design. Give your best hypothesis of what you think you would find out. I'm not looking for a detailed study, but I am looking for you to have the understanding of some type of study design.
For example, perhaps you wanted to study women between the ages of 18-35 and their attendance at a gym, and the corresponding effect on their mood, before and after their workout. You would collect: Name, age, marital status, children or not, working or not, time of day, date of workout, smoker or not. I would have the women fill out a 5 point rating scale, rating from 1-5 (with 5 being the highest) the mood they were in. Compare it to before and after.
Then for my hypothesis - According to data that I have read, I would expect that the moods would be better after the workout. I would expect that married women had better moods regardless, I would expect that those who worked out earlier in the day would have better moods in general. I would be excited to see if mothers fared better or not, whether working outside the home increased mood, and whether smokers had slightly worse moods, with their nicotine use.