Okay, so... you're writing a pre-industrial Fantasy world.
You should probably start by considering things like:
What sort of biomes is the story set in - What crops can be grown natively, and what animals can be hunted or reared for food and clothing? And what building materials are available?
Read books and watch documentaries or youtube videos about life in medieval times in countries that match the sort of vibe you're going for. The Witcher draws heavily from Medieval Poland, for example, while Game of Thrones (or more accurately, A Song of Ice and Fire) is based on the War of the Roses in Medieval England, but has various other countries with a different vibe (like the Dothraki are a bit like Mongols, for example).
Also be aware, the "medieval period" is a really broad period spanning centuries. Early medieval life, armour, weapons and art are very different from late, so you need to also decide on what tech level and cultural sophistication you're aiming for.
Looking at this depiction of the Battle of Hastings from 1066...
And this depiction of the Battle of Agincourt in 1415...
Perhaps illustrates how much development happened in the intervening 350 or so years in art and the technology of warfare as we go from people in long hauberks of mail to plate armoured knights on armoured horses, and ranks of longbows, and the artistic development starts working with perspective and atmospheric scattering.
You'll also want to factor in what magic can do in your setting and who has access, because if magic is widely available, it will probably accelerate technological and social progress, but if it's controlled by just those in power, it could enforce a feudal system.
Basically, go study some history!