This is a hard question for me, officially I didn't decide something was "wrong" and that I was transgender until about 18 or 19, when I actually started to learn what that was (I was very sheltered from "bad" things in my childhood). I had to look back into my youth and find those signs to determine whether I was truly validated in being transgender or not.
That said the reason I questioned to begin with was very obvious, I was never comfortable as a female or in any female roles. The earliest memories I have is when I started to form some semblance of individuality, I didnt play with dolls and girl toys (even though my parents got them for me), I instead played with stuffed animals. I stopped wearing dresses when I was an older child, I hated them. I liked videogames and action movies and "boys" cartoons, when I learned girls likes shopping, makeup, shoes, and were all pretty and dolled up like in movies I disliked it very much, I liked bugs and lizards and snakes and fantasy movies, I liked scientists and warriors and smart people in the media who were most often guys, and was really annoyed when girls were grossed by those things when I wasn't. I had a younger brother I grew up with, so I knew the whole time I was obviously a girl physically, but I rejected all things girly, when I earned the title "tomboy" I was very happy with that because it fit me. I was always very boyish for a girl and didnt fit in with girls much.
Puberty was when the discomfort phase started to get worse. I don't think I ever thought of growing up, it hadnt crossed my mind. I was just a kid. The idea of being a "woman" someday filled me with dismay and I knew I would never grow up to be a woman.I hit puberty late, but when my breasts first started coming in, I felt miserable, I remember looking in thr mirror being confused and unhappy realizing my life is going to change forever and there's nothing I can do about it. In middle school for a female I always had tiny breasts, I was never a "normal" girl, and I had a phase where I wanted them to be bigger because it was weird to not be a girl, I even started to worry that I should get my period because normal girls do that. Eventually I had to get over it and move on, though. When I finally started bleeding, I actually immediately despised it, but it was another thing I had to get used to. Bras, leg shaving, makeup, etc. were all girl things I didnt particularly like but had to do anyways because it was "normal". I still wore a lot of t-shirts and jeans and flannel at this time because I honestly hated to wear frilly girly clothes and the older I got the more types of girl clothes I rejected. My adolescent years were filled with apathy and indifference of my physical appearance, I didnt feel good looking, I was just there, a person. On top of that my mother would force me to be feminine most my younger years and it sucked.
My high school and teen years were filled with me trying to find a style of femininity that suited me because I hated being female so much; went through dark clothes, band shirts, the idea of being scene crossed my mind but never happened. eventually the length of my hair started to bother me, I thought long hair was awful for some reason, I cut it midlength for the longest time because it was the shortest I could get.
Around high school on the internet I learned about a comic (called twokinds) which featured a male character who was actually a female, and that clicked with me so damn much. I was like "Yes!!! This is like me!", I didnt learn until later that it was an allusion to being transgender. I learned about genderbending, which is taking the gender of a character and making the character as if it were the other gender. That was really interesting and I decided to do it with my persona character. I eventually decided I preferred the male persona, in a way changing my persona eventually changed myself.
The idea that I may be transgender started to sink in the more research I did, the most defining moment being when I repeated the phrase "x makes the man" in my head, I was going to change it then paused, realizing why should I? It actually felt good using that gender pronoun for myself. Since then I mentally realized I was male.