So far in the Prologue, my choices don't matter.

[Shampoo]

I find that many of my choices don't matter, and it always goes the same way like for instance when you have to choose whether or not to expose yourself as a detective to Orihime...

I was hoping that I could keep my identity a secret but nope it doesn't matter.

What's the point of giving me the option like the one for Orihime if no matter what I choose, it doesn't matter?

[Firehawk]

If you seriously expected that your very first choices in the prologue of most visual novel will instantly make groundbreaking difference then maybe you need to reassess if what they are to you. VNs are complex stories, the better ones anyway, so you need to treat them accordingly and read carefully.

[Shampoo]

I don't know...

If I was given a choice, I expect that choice to change the game in a meaningful way, especially since the store page advertises that the choices you make changes the game...

It doesn't matter if that choice is given the moment you start the game or after the ending credits, if there is a choice to be had, there is something to be changed by that choice...

If no matter what I choose, the outcome is the same, then they might as well not bother giving you the ability to choose at all.

But then again, that is just me. Still a interesting game though, but it is a bit too slow and I am losing interest because the girl school is boring, I spent all my free time drinking coffee like a good detective haha, and of course, a lot of my choices don't matter and that is disappointing.

[Firehawk]

Sounds like you had misconceptions about VNs and this one in particular, so if you're still in 2 hours and don't like it you could ask for a refund.

[scram]

The whole "your choices matter" thing is a gimmick to me. I get that VNs toe the line between a proper book with a rigid storyline and a videogame that provides the player with agency in how things are done, but this a story that unfolds in a certain way in order to stay internally consistent. Contrast this to a typical romance VN where you choose the girl to take to pound town: the main points of choice are whose route you go on and how their ending pans out. The marked difference in how things go is there, but the writing often ends up contrived and unfocused because you have the scenario writer(s) having to do different stories. It's the difference between 2 or more weaker stories over one strong (and in KnS' case, a very strong) story.
If you want a VN that provides you with the illusion of choice and agency whilst still being a solid read, try Steins;Gate.

[Paris]

It's been a while since I last played Kara no Shojo, but I remember that there are a ton of choices in the game that do drastically affect which ending you get. For example, the option you mentioned as to whether Reiji willingly reveals his true identity to Orihime doesn't drastically change the dialogue in your entire playthrough. However, (Early game KnS spoilers) choosing to tell Orihime the truth is one of the possible flags that can lead to an early bad ending in the game.

If a choice doesn't seem to have any immediate effect on the game, chances are the choice is going to act as one of the long-term flags that impact how far Reiji gets in his investigation and how said investigation ends. TLWiki had a good walkthrough[web.archive.org] that shows which choices affect which ending you get.