The first step in establishment of realistic goals would be to identify the desired end result - what is it that you are setting goals to accomplish? You have to decide what you hope to do before you can create goals that will help you work toward achievement of your final result.
Once you determine what you hope to accomplish, you can start breaking down the steps needed to get there. This is where the "realistic" part of the process comes into play.
If your ultimate goal is to raise enough money to be able to attend college without having to work while you're a student, for example, setting a goal of winning a lottery prize large enough to pay your college expenses is probably not realistic; you may not be old enough to legally participate in the lottery, you may not have the money to buy lottery tickets if you could do so, the odds of winning a lottery jackpot of the needed amount are very small.
However, you could determine realistic ways of earning money and list the steps needed to obtain one or more of those jobs. You could identify ways of investing your wages from those jobs so as to have them earn dividends or interest, thus speeding the increase of your college fund. You could locate sources of scholarships, grants, or loans and could create a schedule or timeline to help you complete application procedures and submit materials in a timely manner.
Once you decide what you want to do, you need to brainstorm as many possible ways of doing it as possible. Eliminate the ideas that are unrealistic because you don't have the ability or equipment or other requirements to make them happen. Analyze your realistic ideas and identify the steps you would need to take to reach them.