I'm trying to practice my writing skills, and I found this one to write about - "our own reality is the only one that matters "
You might want to think about whether you actually agree with this statement.
Your own reality is made up of your perceptions. But other people's realities may differ from yours and they matter as well. You have to deal with people (friends, family, teachers) who may perceive things differently. When that happens, you have to try to make your reality and theirs match up so you can communicate. This means that your reality is not the only thing that matters.
As far as how to write, I am a strong believer in organization. I believe that you need to be sure to make separate points in separate paragraphs. I always tell my students that they need to be sure they state what each paragraph is about and then make sure that everything in that paragraph supports that point. You can't write effectively if you just write down whatever comes to mind without organizing it.
First of all, let me say how nice it is to hear that someone is practicing their writing on their own. This is a wonderful thing for a writing teacher to hear!
To represent this passage, I would suggest thinking of a time in your life in which you had a misunderstanding with someone, or your perception of an event was vastly different from someone else's. For instance, you might have had an experience in which you had a disagreement with a friend. They might have said something to you that you took differently than they meant it. You could explain the impact of that moment - perhaps you didn't talk for a few days, or you didn't go on a trip with that person as a result.
Later, you may have found that your perception and his or her perception was incorrect or skewed. The important thing is that it wasn't "incorrect" at that time. That is where the reality comes in. What was said at that time is what was real to you. In a similar manner, this is why rumors are dangerous. If a rumor is untrue, but enough people hear about it and spread it, it becomes true in people's minds and the impact of that rumor is very true.
I think it's important for this question that you make it personal. You can talk about your own dreams and your own wishes and about how that is the only thing that matters to you. If it's real to you, it's real!
I've attached a link to "How to Write a College Essay" that I think you might find helpful. It discusses personal writing.