Create: Defensive integrates into your world from the very beginning, with many materials and tools available within your first few in-game days.
When starting out, your focus should be on upgrading your combat capabilities beyond vanilla gear.
Gather Copper and Aluminum, which generate more frequently than Iron, and use these materials to craft early-game weapons, armor, and tools.
These provide a noticeable upgrade over Stone gear, but are still weaker than Iron-tier equipment.
Once you have basic gear:
Begin replacing your vanilla combat equipment with Defensive's alternative medieval weaponry. Many do different things, have different cooldowns and damage values, or cater to specific playstyles. Experiment with different weapon types to find what fits your playstyle.
Keep an eye out for rarer materials like Silver or Platinum, which unlock stronger equipment.
A lot of materials can't directly be crafted into new tools or armor, they need to be Smithed just like Netherite. Silver can be applied to Iron, Platinum can be applied to Diamond, and Electrum can be applied to Netherite. These smithed sets actually apply useful boosts to the player, like Silver applying a small regeneration boost if the whole set is worn.
There are a lot of materials and ores you'll find in the underground and it might be a little overwhelming, but all of them have uses and are worth saving if you find them.