Try out AI Dungeon which is an app that has it all set up for you. It's free to try, but pay for access to the best model. Last I checked there were some privacy concerns, so assume someone might read your adventure and don't tell it your real name or anything.

I've personally been tinkering with KoboldAI which is a way to run the models yourself, if you have access to a beefy computer or cloud computing workspace. This has the benefit of being free and controlled entirely by you, but requiring you to choose a model and giving you the opportunity to change your own settings might be a benefit or a drawback depending on how much you enjoy tinkering with your toys before (and while) playing with them. The models that will run on my RTX 3080Ti seem generally not as good as the paid tier of AI Dungeon, but I might also not be doing it right? It's hard to tell. Futzing with parameters and trying to divine what impact they're having on the output is still fun for me, though.