You can put the major that you think you might like to study and no one is going to hold you to it, I promise! But it can help the college get to know you better. You are also welcome to put “Other” if your target major is not listed. I am not a huge fan of “Undecided” unless you can list the two or three things that most interest you at this time. I think listing undecided means you pass up a chance to say something about your mind, heart, and passions.
In that section you can also state your future career idea, if you have one, and the highest level of education you plan to receive. Your generation is most likely going to attend grad school, too, so I would list MA or MBA or JD, etc. Unless you know for a fact that you are not going to go to grad school, in which case you can of course just list BA.
Hope this helps!