First you kill monster by equipping weapons there are many different kinds of weapons axes, spears, hammers, but the best to start out with is a sword. Either one-handed sword or a two handed-sword. One-handed swords can be swung a few times without stopping. It's called combo the more combo a sword has the more times you can swing it without having to stop. Two-handed usually have less combo and the swings are very slow. It also takes a lot of RP if you run out of RP you'll start losing HP whenever you attack and eventually you'll kill yourself. The only upside to two-handed swords is the attacks are very powerful. But it is better to start with a one-handed sword.You can increase your HP by leveling up. Every monster you kill will give you a little more experience when gain enough you will get a level up and certain skills and HP will go up a little. If you want to befriend a monster make sure you have a Pet Glove (or maybe it's called a Friendship Glove) and a monster barn. I think Tabatha gives you the pet glove and I know you have to hire Camus to build you a monster barn. Monsters can not be easily avoided and if you want to beat the game you're going to have to do a lot of fighting. In fact the whole point of going into caves is to destroy all the machines summoning the monsters then beating the boss.