How do I turn Redstone into Redstone Dust in Minecraft?

There is no item actually called “Redstone Dust”; this is a colloquial name for the  Redstone item, because it has the appearance of a pile or trail of powder. If this is what you have, then you already have the “dust” and you can use it as you have seen discussed.

If you have a stone-like block with red inclusions, then you have  Redstone Ore. This is not usually the case because it requires an pickaxe enchanted with Silk Touch to obtain without breaking the block. To obtain Redstone from Redstone Ore, simply place the block and then break it with an iron or diamond pickaxe that does not have the Silk Touch enchantment. (You can increase the quantity obtained by breaking it with a Fortune enchanted pickaxe.)

You already have redstone dust. You can place that on the ground to make redstone wire which can be used to make all sorts of different circuits which can be used for different purposes. You can always pick up the redstone wire and put it back into your inventory. When you do that, it is called redstone dust again. Here is the wiki page on it.