Luckily I have been taught what to do. As you grow up and learn more and more science you start to learn and make decisions by yourself. When you start a new job as a scientist you are always given some help as to what you are doing, so you dont find it too hard!
You always work with people who do the same thing as you, and go to big meetings and conferences with lots of other scientists and through talking with them you might get an idea of something new to do! Scientists never stop learning how to do new things and everything you have learnt all through school helps you when you grow up so that you know what to do!