How should I structure product data so customers can filter products properly?

Start with a consistent taxonomy, then define the attributes that actually distinguish products within each category. Use controlled values for fields such as colour, material, size and product type instead of allowing every product to use different wording. Expose those attributes through filters only when they help customers narrow the assortment. A good filter system mirrors real buying decisions: shoppers should be able to move from a broad category to a relevant subset without needing to understand your internal catalogue structure.