You will need a carting harness - there's a few on ebay or you can try sledding outfitters who often have many different types of harnesses (skijoring, sledding, carting, tracking)!
Here's an example: http://www.akkosports.com/contents.htm
Directional training is pretty easy - I taught my dog to target for that.
You you can also teach them directions by saying "haw" or "gee" when you are about to turn. For instance, if you need to turn left up your driveway say "haw" - eventually your dog will come to understand what it means.
Basic terms he'd need to know:
"Ready...Hike": get ready and go
"Easy": slow down (and stop pulling)
"Gee": turn right
"Haw": turn left
Your dog will probably need different commands for sharp turns or just slight direction changes.
Aside from the obedience and training of new commands, you also need to teach your dog how to pull. I'm sure I didn't do it right but once Dodger was comfortable with the harness, I simply attached a log to the line, and encouraged him to pull (I had him on his leash). At first I walked either ahead or to the side but I was using commands like "hike" or "go" to keep him moving. Then, I slowly made the transition to walking behind him and saying "hike" (and had a double ended leash to hold onto)...
At 6 months, you can certainly train all the commands and teach him how to "hike" in a harness (with you in behind, holding the double ended leash but no weight to it)... But it is best to wait until he is about 18 months before having him pull an empty cart.