Soldering Iron Tips Stop Working/Melting Solder

Hey everyone,

I'm a complete beginner in the world of soldering, and I'm facing some frustrating issues with my soldering iron tips. After just a few minutes of use, my soldering iron tips can't melt the solder anymore.

I always keep a sponge nearby and make sure to clean the tip frequently, just like I saw in tutorials. However, the tips still look a little "burned"/ black. Am I doing something wrong? Or could it be related to the soldering equipment I'm using? I bought a $20 kit on Amazon just to practice.