Do I have to do what I write in essays?

If you are talking about taking certain college courses, activities, etc, then no there is no obligation whatsoever to fulfill those things (assuming, of course, that we're not talking about supplement essays that are for actual, explicit commitments). Your supplement essays are written to get a sense of what might interest you, not to lock you into a four-year path from which there is no escape.

Having said that, it seems that you already have a sense that whatever you are about to signal interest in might not actually interest you as much as perhaps the essay will suggest. At the least, this would be poor form. But (more?) importantly, without knowing any of the specifics of what you are writing about or anything about you as a candidate, if you are writing about something that you feel you might not ultimately be super interested in or want to commit to, there is a chance that you are writing what you think the admissions committee is going to want to read, instead of an unfiltered, genuine exposition of who you actually are. This is almost always going to be a bad move for three reasons: 1. Admissions officers are very good at picking up on this. This is bad for obvious reasons. 2. Even if they don't pick up on it, there's a big likelihood that the tangibles which mark your more pertinent interests (extracurriculars, awards, academic performance in certain disciplines) are going to clash with your essays, meaning that your essays aren't going to line up with the rest of your profile and you will look like an inconsistent candidate with inconsistent interests (if you write about wanting to be pre-med, but your heart isn't in it and essentially nothing else about your academic profile supports that statement, even if admissions officers believe you all you are doing is putting yourself into consideration for a field that you are going to be at a disadvantage in), and 3. if, somehow, you get past the first 2 obstacles (which, to be clear, I would not pin your hopes on), you've succeeded in putting yourself on track for a four-year experience that you might not enjoy and, depending on whatever school/major/program is in question, might not be super easy to get out of.

Discharging that hypothetical, if this is indeed genuine interest/potential that you're just not sure about following through with, then by all means write about that course, activity, or major -- perhaps signalling, if appropriate, that it is a flexible interest -- but you have no obligation, legal or moral, to follow through on what you have written about. Colleges know that peoples' interests change, and they're okay with that!

Definitely not! Admissions officers just want to know what you MIGHT do—they're not going to hold you to it.

I take it you are talking about your aspirations for the future? i.e "I am thinking about joining a club" or "I want to get a major in Business"

That is completely fine and you do not have to actually do those things. They will not track everything you do to see if you fulfill all the goals in your essay. They will not require you to actually declare your major until you are actually enrolled in the college.

That being said, you shouldn't lie either. Just don't get super stressed out about doing everything you write down.

I’d avoid it as they can easily verify it by checking your transcript so only so it if you are locked into the class and are certain that is an activity you will do as saying in Band I have done xyz without taking band is a big no and may even be an auto ejection from schools with 80% admit rate much less the ivies.