Conflict is the essence of any good story and warfare is the essence of conflict. War generally means weapons. Long knives, daggers, broad swords, and hunting knives all have distinctive purposes and certain characteristics. The weapon needs to fit the character. A small woman swinging a broadsword is not believable. Absent incredible magical powers, this petite heroine won't be able to handle such a heavy sword effectively. But what's more, she won't be able to wear that long sword in a scabbard at her waist because it will drag on the ground. A stout strong woman might be able to wear it in a back scabbard, but given the general lack of upper body strength of women, will she be able to draw the weapon effectively over her head? It might just be easier for her to wield a long knife.
In Daermad Cycle, Ryanna is a half-kin nearly as tall as a Celdryan male. Kin are not human, so they are stronger and faster, plus she dresses like a man, so her opponents don't realize she's a lass. Her best skills are quickness and surprise and she can be overmatched by a larger male or if the fight goes on longer than she intended.
If you're writing an urban fantasy, learn about handguns. Go visit your local gun shop or take an NRA shooting class. My instructor had about 20 or 30 different weapons for us to try. While I was familiar with some of them just by virtue of growing up in Alaska's gun culture, there were others I had never handled before.
What do you know about death and dying? Most people in the Western world never see anyone actually die. Someone suggested that the state medical examiner's office could help with details on death. That was not a good option for me because that office is in another city, but I interviewed a few combat veterans from my church and the cop who lives up the street, plus I have hunted animals, so I was not wholly unfamiliar to begin with. Realistic details amps the believability of the story.
What your characters wear is just as important as their weapons. Traditional fantasy characters generally wear clothing typical of the Middle Ages. You can google those images these days. The more research you do on the era that you're setting your story in, the better. You may even be surprised at some of the "modern" conveniences that appeared before the Middle Ages. Ancient Egyptians used eyeliner, eyeshadow, creams, oils, and moisturizers. China has used fireworks for centuries. If something existed somewhere prior to the Middle Ages, you can feel safe introducing it into your story. Be sure to check your facts and talk to experts in the field if you can. If you are using an alternate universe as I do in Daermad Cycle, you can mix and match eras and cultures, just be sure to have less technologically advanced societies be surprised and even frightened of the capabilities of others.
Nothing adds authenticity to a story quite like bodily functions. Allow your characters to do real things like eating, sleeping, and urinating. In a world without screens on the windows, there will be flies circling their heads while they eat and bugs bite. Horses do too. When a woman spends days tramping through the woods, her hair is not going to be bright and shining and her lips won't be like pink petals -- unless of course she's magical -- or dead. And if she's ever called upon to use that long knife, those dagged sleeves on that dress are going to be a major impediment to skillful swordsmanship.
The point is that magical worlds are believable only when they are respectful of reality. The more real you make the world you are creating seem, the more readers will respect consistently applied magic in that world. Remember your world won't be real to your readers until it's real to you. Invoke all five senses and stop to think from your character's POV. Always remember, its your world, but it also needs to become your reader's world so that they will willingly suspend their disbelief.