>>96303504
If like most people you were introduced to rpgs through dnd, I would recommend scarlet heros.

It is a dedicated solo OSR system, meaning compatable with old dnd publications like keep on the borderland (dungeoncrawl) or isle of dread (hexcrawl), or do your own adventure, which it has the basic solo tools for in the book. It was purpose built for solo play and adds elements to make it so that a single pc can survive and adventure through scenarios meant for parties.
This is done through 3 basic additions:
1. damage reduction: there is a damage reduction table for the pc
2.extra attacks: you get an extra attack action against things equal or weaker to you so you can deal with minions easier
3. meta currency: you can spend an increasing amount of health points to negate a consequence

there is also a lot of solo resources for osr stuff in the likes of dungeon magazines.