Skeletron is the boss blocking access from the dungeon. Killing him will grant you access to the dungeon, you will be spending a large fraction of your time here come Hardmode. However, before we kill this boss, you may have noticed a video above explaining how to make obsidian skin potions. This is important for melee, as the armor and weapons we will be using come from hellstone.
This stone is only found in lava, however using it, obsidian, and a hellforge will allow us to craft hellstone armor and the Volanco sword.
Spawning Skeletron is extremely simple compared to the previous bosses. Come nighttime, head to the dungeon, and an old man will be standing there. He will have a curse, and lifting him of it will spawn skeletron. You have to focus his hands first, as hit head won't take much damage until both are broken. After destroying both hands, skeletron will begin spewing skulls at you, but without his hands, he becomes much less of a threat. Defeating him won't grant you much, besides access to the dungeon. You are free to explore, but there aren't many things inside for the melee class just yet.
The Wall of Flesh is the final boss of pre-hardmode. It is usually the hardest boss to fight, especially for melee only. The video I linked above gives you a small guide on how to create an arena for this fight, along with some other tips if melee only proves too hard.
Summoning the wall of flesh requires the doll you get from the enemy above to be dropped into lava. The Wall of Flesh will constantly approach you, summoning minions, and shooting lasers out of its top and bottom eyes. The best way to fight it is with bee weapons from a side boss, jester arrows, and or the hellwing bow. But as a melee class, the best way is to play safe, you have more time than you think, and if you get low, back away from the boss, and focus on dodging and healing. However, once you manage to overcome this astronomical threat, you will have beaten pre-hard mode, and that is a feat worth celebrating.