One of the reasons sashiko is popular is that it doesn't require much equipment to get started.
The most important things are fabric, thread and a needle. Cotton fabric is often used because it is easy to sew through and holds its shape well.
For the actual embroidery, sashiko thread is usually used, which is slightly coarser than regular sewing thread. It makes the stitches clearly visible against the fabric.
A sashiko needle also makes the work easier because the long needle makes it possible to pick up several stitches at a time.
With these simple materials, you can start trying out the technique and gradually explore more patterns and projects.