You need to create a texture using a texture template and a image editing software like PaintShop Pro or Photoshop. For Michaels Clothing Pack, I would believe that the texture template for e.g. the shirt can be downloaded from DAZ. There are plenty of free textile-like textures out there to download that you can use as a basis and apply to the template. My experience is that it's more important to have a good bump map in order to create the impression of fabric in the render. Fabric usually has a very uneven surface and that must show in the render. Sometimes you can actually have a solid color on the clothing item and just apply a bump map with a pattern from a fabric. You can easily make a bump map in Paintshop Pro or Photoshop by converting a hi rez picture of fabric to a grey scale picture.