When in caves, monsters will attack you. At forst, you can just attack them with your hoe. Later, you can go to the Blacksmith and he will forge you a sword. After building a monster hut, you can keep monsters on your farm. Talk to Bianca and she will give you a petting glove. This will allow you to attempt to befriend monsters and take them back to your farm. You can get the monsters to help you in the field or help you in journey's in caves. You can later get a sleeping bag to restore stamina while in caves.