Locate a plains biome, wait until night, so the boss can spawn, once Toriel spawns you can then kill her for a Snowdin Boss Key, once you obtain the Snowdin Boss Key locate the Riverman Trader (he spawns throughout most places in the overworld), trade with the River person to get the craftable to fight the next boss, repeat this same process once you kill that boss, repeat for the other bosses, or if you like to be more adventurous you can also find the bosses after Toriel in the overworld in their respective spawns (Papyrus in Ice or Cold biomes and Undyne in swamps or mangroves), once you get geared up after fighting the bosses you can then start getting souls!