1) Like all businesses, you start with lots of background research.
Google FTW

2) Write out a business plan. Define your goals, find what you will need to meet those goals. Buy a spiral notebook and fill it with your goals, ideas, concerns, thoughts, just write it down so you won't forget it and will be able to review your ideas for the business for the past month or so to find those few really good ones.

3) Make sure this is something you REALLY want to...
"The Supreme Courtâ??s recent ruling in Gonzales v. Raich, 125 S. Ct. 2195 (June 6, 2005), upheld federal power under the commerce clause to bar medical marijuana. The court ruled that federal drug laws override initiatives such as Californiaâ??s Proposition 215 that allow limited marijuana distribution for medical purposes. As a result, federal law enforcement agents could theoretically shut down all growers and medical marijuana clinics that operate according to these state laws. But they havenâ??t." 420 LAWYER - Planning on starting a California Proposition 215 Medical Marijuana Dispensary? Then Read On...

Figure out all those little tiny imensely important things like have $200k for legal fees, having your family know you sell cannabis for a living, and so many other things I can't even begin to think of. Do your research, you are officially the 1,000,001 person to ask about this, so look around the internets!