It would be grown by government approved vendors and taxed. Private growing would likely be illegal. Sales by non-government approved vendors would likely be illegal. I'm certain the government would allow allow a single FDA approved variety: you wouldn't see the higher powered varieties or growing techniques. It would more highly regulated than cigarettes or alcohol. I have no clue about the number of how all this would work out, but it could be more expensive than it is now.
If you get caught with a cheaper variety, or more powerful variety, etc, then what? Probably the same thing that happens today. After all, you can get arrested for carrying around prescription drugs that aren't properly labeled; you'd probably see the same thing with government marijuana. Moreover, you can then be charged with tax evasion.