Well, as far as I know you can not get AutoCAD for free (at least legally).
But there are options, you can get free alternatives that work very well, these are my favorite CAD software replacements:
For 2D CAD you can use Draftsight, it’s free -as free beer- for personal use, it can open/save DXF aswell as DWG files.
For 3D CAD you can use FreeCAD, it’s free -opensource- , it can open/save STEP and IGES files between others.
For Architectural CAD, CYCAS is a very good option.