Making clothes for Poser and/or DazStudio you'll need:

1. A modeling program to create the clothes (Hexagon, Blender, Wings3D, Maya, etc.)

2. A way to group the modeled object to the same groups in the character your are creating the clothes for. (you can group in Hexagon, Blender, etc. or you can purchase an add-on like Auto Group Editor)

3. Poser or Daz Studio Figure Setup Tools (Skeleton Setup) to transfer the character skeleton (rig) to the clothing item you created.

4. Poser or Daz Studio Figure Setup Tools (Joint Editor) to modify/create falloff zones to make the clothing move with character poses correctly.

5. A 2D program to create and add textures to the clothing object you created. (Photoshop, GIMP, Paint Shop, etc.)

6. Optionally, a software tool to transfer morphs to your clothing item. For Daz Studio, this will require the Advanced Figure Setup Tools, or you can purchase add-ons to complete this process.

Those are the basics. Each step will require a lot of learning and experimenting to perfect the skills necessary.