Pick a class early
Melee, ranged, magic and summoner each have their own armour and accessories. Choosing early means you can use the right armour set the moment it becomes available instead of spreading materials across four sets.
Build an arena before the first boss
Several rows of platforms with campfires and heart lanterns turn a hard boss into a manageable one. Arena preparation is the largest single difficulty lever in the game.
Follow the boss order
Bosses are tuned in sequence and grant access to the materials for the next tier. Check the in-game bestiary or the guide NPC for what to summon next rather than skipping ahead.
Mobility matters more than defence
Boots, wings and a hook let you avoid damage entirely. Prioritise movement accessories over incremental armour upgrades.
Build a storage wall
Chests sorted by category, near your crafting stations, save an enormous amount of time. Terraria has a very large number of materials and no auto-sorting.
Use buff potions
Potions are cheap relative to their effect and are the difference between a close fight and a comfortable one. Keep a small garden of the herbs you use most.
Explore the underground for accessories
Chests in caves contain the accessories that define builds. Systematic caving is more reliably rewarding than digging randomly.