There are programs to convert a scanned image to dxf http://www.scan2cad.com/ but none that I know of for free.
You say your skills at Autocad are not good does this imply that you have Autocad?
If you do have it then it can import scanned drawings over which you can draw in cad.
Mind you in some respects I can sympathise with you as getting actual g-code from drawings is not all that easy!

peter