I was in the same situation last year, with the same questions. I am from Virginia as well, so I can help answer your licensing questions.
You can obtain a license from your County Commisioner of the Revenue. If you are going to do business under a company name rather than your own, the first thing you need to do is go to your local courthouse and file a certificate of assumed or fictitious name with the Circuit Court. This cost me $10. Once you have that form completed, you need to go to the Commisioner of the Revenue office and file for a business license. The cost of the license varies depending on the amount of income you are expecting. If income expected is under $10,000 it will be about $30 for the license. I'm not sure what county you are in but go to your county's website and you should be able to download most of these forms online and also obtain more info.
Once you have your license then the next step is getting insurance. Shop around with different companies! i have mine through Nationwide and it is reasonably priced. Definitely get insurance to cover your @##.
As far as equipment, don't over buy the size you need. I see a lot of people running around with a ztr on their trailer. I always wonder why everyone thinks you need a ztr. A good quality
walk behind works just great for starting out. Don't go cheap because that just menas you will be replacing it in a few seasons costing you more money. Depending on your yard sizes I would probably look at starting with a 36". If you do mainly residential stuff, like me, a 36" walk bahind will allow you to get through gates into the backyards. An oversize ztr won't, which means you pretty much spent that money for no reason. Go to different dealers and demo the different brands. Find what is most comfortable for you. Personally I like my Emark. Next think about options on the mowers (floating vs. fixed, etc.) I got a floating deck simply for the ability to easily change deck height, and this has been an advantage for me since I have a few customers that ask me to mow shorter than my others.
As far as advertising, I just simply ran a classified ad in the local paper. It got me off to a great start!
Hope that helps!