Actaully its NOT possible to convert a DWG file to STEP file in Autocad.... But it can be done using Solidworks!!!!!
Open Solidworks and load the DWG file which you want to convert into STEP format..
Now click on Import a new part as option and ensure that 3D curves or model option is enabled and hit Next option
Now its time to select the Units of imported data, view orienation and wether you want White background or not and press Finish
Next step is to click on Save Button
Now give a name for your File and Select Save as type to STEP AP214...
So now you have converted the DWG file into a STEP file...