You start selling by starting to sell. Ebay is the easiest entry for beginners. You could even start by selling extra stuff you have around the house, to learn the ropes, before you start sourcing products.
I actually started back in the 1990's selling my oil paintings. What I quickly learned was how important it is to standardize your packing materials. Have one size envelope or box that fits anything you want to sell. Next printing labels is a real hassle. Spend the $70 upfront and get a DYMO label printer. 6x4 is the perfect size.