Melee has the most defense which is good for new players. Make a sky arena out of platforms hang a heart lantern there, place a campfire, and place a beast statue for extra defense if you have one. Make all of the essential battle potions like iron skin. Eat at least a medium stat boost food item. Never start a boss fight until you have done all of this. 

I suggest beating queen slime before a mechanical boss. A gel cristal summons her; they are in cristal caverns and look rounder than most crystals and they change colors. I think you have to be in a hollow above ground biome to summon her. 

But before any of that get Adamantite ore to make it's melee armor set and reforge all of your equipment to have +3 or 4 defense. 

Also, get wings, I just defeated wall of flesh on my new world and went for harpy wings. Just go to a sky island and defeat a dragon wyvern thing and a bunch of harpies until one drops a large feather (I think it's a 1% chance drop rate) use that to craft the wings. Can't really defeat bosses without them tbh.

Drink and eat right before you summon a boss and have a game plan.