I was searching the site for another subject and found your post here that had not received an answer.
here's how to get started:
1. contact your local Ag extension office or your state Agricultural Department and ask them to put you in contact with the Agricultural inspector for your region.
2. go to your states AG Dept website and see if you can read about the regulations and requirements for a nursery license.
3. If your state has a nursery license application online, download it and print it out.
4. contact your city and/or county to find out whether you can have a business at your residence and how to get a business license.
5. you may have to get a zoning variance if you plant to have customer traffic and abide by regulations such as insurance, ADA access and restrooms etc if you do.
6. once you have cleared that you can have a backyard nursery at your location (you may have to be 'ecommerce' only with a once or twice annual garage/driveway sale depending on your zoning), file all the appropriate forms.
7. get a state resale license or whatever it may be called to collect and submit sales tax if your state has a sales tax.
8. go to your bank and get a business checking account and credit card. any reputable wholesaler you will want to do business with in the future will require you to be a real business before they will give you true wholesale pricing, some won't even let you in the door without proper documentation.
9. talk to an accountant and set up business software. this can be as simple as some excel spreadsheets or as complex as a specialty nursery or grower program.
10. purchase the domain for your business, even if you are not doing ecommerce now, you might want to reserve the name for the future.
11. grow and have fun ---- and sell!
this is probably not all-inclusive, join some of your local garden or plant societies and there'll be some backyard nursery owners - talk to them!