If you buy Crumplebottoms Garden Shop or Goldblooms Grocery Shop and place them on any public lot you will need to click on it to Hire a Vendor. After paying a fee, the lot will be staffed. You can't use it to sell your own products though. The same applies to any stalls whether it's food stalls or produce. There's a photo here where I've added stalls to a public lot in my Newcrest retail park. They're food stalls but it's the same principle.
To sell your produce to the public you need a selling table, also found in Activities and Skills - Outdoor. Like the Ground Floor Entrepreneurs Table from Eco Living or the Come and Get it Street Store from City living. You can place them on a public lot and use your inventory to stock them and then see if you get any custom. Personally I find it a bit slow and I don't usually get much custom even with high quality items.
If you have Get to Work you can sell it in a retail lot like I do. The second photo attached here is a Garden Centre run by my Cottage Living farmer. I've just added items to tables and shelves and set them for sale.
You can also obviously just sell them from your inventory and you can use the Register with Ministry of Labour option on your phone to give your Sim a job title. So mine is a Farmer and Greengrocer. This makes it feel more like a career and can be done for any career you want to create for your Sims.
On the subject of helpful sites I like Carl's Sims Guide. I'm pretty cautious about websites incase I get a dodgy one but I like his, especially for collecting tips. It can be frustrating as a console player I find, because so much is geared towards PC players.
