You need to consider what you want to major in early on in high school. Having something in mind does not mean being one hundred percent certain of what you want to do. You should simply have an idea of what you want to study; ideally this corresponds with your passion. The same advice that I gave above for activities applies here, about not trying to fit a certain template you think exists for each school. If you want to be an engineer, but would like to major in political science as an undergraduate student, you can still apply to MIT as a political science student. You should however discuss the choice in your essay or interview, so that it doesn’t look random. As long as you can explain the “why”, the “what” does not matter. You should have researched the major you choose, so that you can answer any relevant questions during an interview. Since you have the option to select the major for each individual college, you should make sure the institution offers that major before choosing it.
If you truly have no idea what you want to do in college in terms of your major, do not force yourself to put something else down. It is all right to say that you have no idea what you plan on doing. This shows that you are being genuine and that you are still growing as a person. You should exhibit skills in various areas and make sure that you are still showing an area of passion.
There are a few things to remember when applying as undecided. The first only has to do with colleges that have specialized schools for various disciplines. For example, if you are interested in engineering and a school has a separate engineering school, it would be a good idea to decide in that instance. Also, you should make sure with the school that the major you indicate is not binding. If it is, applying undecided will give you a little bit more time to decide. Most liberal arts institutions will not have you take classes for your major until the second year of school, and so choosing a major (or not choosing one) should not affect your college career.