It depends on what you’re making. You definitely need a general 3D program like Maya, 3ds , or Blender. After that there are particular tools that can help in certain situations. Mudbox is for sculpting and for painting textures directly onto the mesh.
And as someone said earlier, if you’re a high school or college student then Autodesk has a site where you can register and get free educational licenses of their major software programs for free.