Erick, that part shouldn’t matter…
Dadonenn, on the materials window, look at the box with the material list. (It’s the part marked 3 in the link that Stan provided) The first option is the “default” material, which anything not assigned will default to.
To add a material slot to the list, click on the plus sign + to the right of the box. Click on the “New” button (or select an already made material from the drop down list next to the “new” button) in the area marked 2. Now, you have 2 materials assigned to the object. All that is left is to tell it which faces should have this new material. In edit mode, select the faces that you want to have the new material. When you are in edit mode, there should be a row of 3 buttons between the areas marked 2 and 3 on the image from the link above. One says “Assign”, one says “Select”, and one says “deselect”. Click on the one that says “Assign” to assign the new material slot to the currently selected faces. If you want to see what faces are assigned to a given material slot, you can choose the “select” option to add the faces to the selection. Choosing deselect would remove the faces assigned to a given slot from the current selection (but they would still be assigned to that material slot).
It’s probably a good practice to explicitly assign materials to a given material slot, but as I said, any “unassigned” faces would take the “default” material, which is generally the one at the top of the list.
You can remove a material slot by clicking on the minus - sign under the plus + sign.
Hope that helps!