Creating a tabletop RPG is a massive creative undertaking. Here's a structured roadmap:
Read widely: D&D 5e (mainstream), Blades in the Dark (fiction-first), Mörk Borg (minimalist), GURPS (simulation), FATE (narrative), Apocalypse World (PbtA engine), Troika! (weird), Lasers & Feelings (one-page). Steal mechanics, remix, make yours.
Start small. Finish a one-shot. Then expand.