These are all bosses you have to fight, how to spawn them, and what you should do to fight them. Skeletron is not needed because mechanical skulls can rarely drop, although I would recommend defeating him.
Wall of Flesh: Spawned by dropping a guide voodoo doll into lava. You will need a guide to make it work. You should build a bridge across lava and break holes in the obsidian houses.
Destroyer: Spawned by using a mechanical worm. You should make a big box with campfires and maybe a heart lamp.
The Twins: Spawned with the mechanical eye. You should use the same box for all 3 mechanical bosses (Destroyer, Twins, and Skeletron Prime).
Skeletron Prime: Spawned with a mechanical skull.
Plantera: Spawned by breaking bulbs that appear in the jungle after defeating the mechanical bosses. mine out a large space around a bulb and put campfires and a heart lamp in it. Make shure the bulb isn't near the underworld or surface.
Golem: Spawned by using a lizard power cell at the lizard altar. To get there you use the key from plantera to get into the jungle temple. lizard power cells can be dropped by lizards or found in chests. Put walls next to the lizard altar because he can't go through walls. Also break all statues and traps in the room and make a wall so that other lizards can't come inside the room.
Lunatic Cultist: Spawned by killing the enemies that spawn outside the dungeon after defeating Golem. Make a platform at the dungeon or make a box around you because his attacks can't go through blocks except his phantasmal dragon.
Solar Pillar, Vortex Pillar, Stardust Pillar, and Nebula Pillar: These are sometimes not considered bosses, but I put them in my list. They spawn at different places in your world after defeating the Lunatic Cultist. They have a force field that will go away after defeating 100 enemies that spawn near it. A tip for crawltipedes: circle around them and hit their back where they can only take damage.
Moon Lord: Spawns 1 minute after defeating all 4 pillars. Make lines of platforms at your base and make a row a blocks for a shield for his phantasmal deathray. Put lots of healing things around the arena and use potions and food like pumkin pie. When I defeated him, I did it with 2 of my friends. You (and maybe your friends) should use full beetle armor and solar eruptions.
Here are links for how to craft the mechanical boss spawners: