Lean on existing designs, look at other games and take mechanical concepts from them. It’s much harder to invent new mechanics from whole cloth when you’re inexperienced.
Some useful references:
Alone Among the Stars
a mechanically straightforward single* player journaling game, in making your own you will write a lot of thematic and flavorful directional text. There’s also a wealth of games based on AAtS if you want to see some spin other people have tried
Lasers & Feelings
A one-page multiplayer adventuring game, in making your own you will assemble several lists, the specifics are up to you but the basic format is a 4d6 plot generator (4 lists of 6 items that you can roll on to madlib an adventure together), and then additional lists for character and context, which don’t need to be rollable. There are many L&F based games, but I don’t have a convenient list of them to link you to. I’ll just point out Oh Dang! Bigfoot Stole My Car With My Friend’s Birthday Present Inside
Honey Heist
Similar to Lasers & Feelings but with a bunch of the bits shifted around a little, the comparison is pretty useful, and likewise there are many games based on or inspired by Honey Heist
Jared Sinclair’s 6e
There’s a lot of games named 6e, in reference to D&D 5e, in part because of a jam to make 6e games, which Jared Sinclair’s 6e came out of. Because of the D&D association JS6e is much crunchier than the things I listed above, but it’s still not that complicated and there’s a bunch of other people’s spins you can look at. You would probably want to write a collection of playbooks, but there’s certainly examples of people doing more ambitious additions.
The Ground Itself
This is a game not about playing specific characters, but about the story of the place characters are in. You draw cards and then process questions, detailing the place, the people, and events across some timespan.
In each of these, making a game(/etc) inspired, influenced, expanding, or based on it involves understanding the shape of the game and making new text to fill it with. Some of that text is more mechanical, but in writing the text you are seeding a world and sketching characters. Players will use your text in play to produce specific worlds and characters.