When you assign the first material, I think the whole object gets it. But then you add a second slot and add a material to it, then you can select which faces to assign them to. So probably you are just adding one material, the black one and the whole object is turning black. You can just change the color to whichever you like and then add the second slot and select the faces you want that material to have. Remember to go back in the lesson if you get stuck and see what the teacher does, chances are you made an error along the way.
Hi Adeoluwa JJiyuuda ,
It actually looks like you only have 1 Material (Black), so as Omar mentioned, the whole Object gets that Material, because there is no other Material.