Ok, so I'm running around and a hammer pops up saying my armor durability is below 30%. How do I fix this? I tried getting new armor, but it still said it.

At every shop keeper theres a repair button. click on that and click on the gear you want to repair

when you go to a shop there should be 2 hammer tabs on the bottom left side of the shop menu. 1 repairs all of your item that you are currently wearing, and the other repairs 1 item of your choosing.

of course the more broken your weapon is, the higher the cost is to fix it.