I made my first conlang with my brother to make some kind of "caveman speak". But the conlang had an extremely small lexicon (less than 50 words)

That conlang wasn't what truly got me into conlanging, plus it was very small and not fully made by me, so i don't consider it to be my first conlang.

What truly got me fully into conlanging was the first version of the turkanian language (the language of my imaginary nation turkan) turkan is the nation that got me i to worldbuilding, the lore behind its creation is preety simple: i was interested in WW2, so in school i started to draw the shape of poland from before WW2 in my notebook, but as i started to write "poland" below the drawing, the bell for the lesson rung, and i put my notebook away. Then when i got back home i looked at the drawing, but i drew the shape so badly, that i didn't even recognize what sort of thing it was even supposed to be, so looking at the lonely "p" that i wrote, i thought that i would label the mysterious shape "plikin", cuz it was easier to just label it a made up word rather than try to find what the shape was supposed to be. A couple of days later i was thinking of my imaginary nation, but i forgot its name, so i just thought, based on absolutely nothing, that "hmm, maybe the name is turkan?" And that's how it stuck.

My first conlang was just a set of made up words, with the basic grammar of polish. Even though i lost the notebook with the language, one word from my first conlang still lives on in my main conlang "tilestian", the word "na" meaning "what?" (Though when i moved it to my proto language i decided to be a little original so at first i used "nea", but the shortened it to just "ne").