You can start seedlings in flats, soil cubes or in individual containers. Many gardeners prefer to start seedlings in open flats or in soil cubes and transplant them in individual pots after the first set of true leaves appear. Individual containers can be homemade or commercial pots. Yogurt cups, styrofoam egg cartons, bread pans and cottage cheese tubs make excellent containers. Be sure to punch a drainage hole on the bottom of each one.
Commercial pots are an additional expense initially, but they offer some advantages.
Square plastic pots take up less space and often come with trays to hold them together as a unit, or pack. I prefer this setup, especially when shuttling the seedlings outdoors and back during the hardening off process. The soil cubes allow for superior root growth and keep transplant shock to a minimum when handling delicate root hairs. This is one of the rare times I purchase commercial potting mediums. Seeds contain enough nutrients to nourish themselves through sprouting, so a seed-starting mix does not have to contain nutrients.