THERE ARE OTHER people who can show you step-by-step how they start tomatoes from seed, but I have two little secrets: 1, APS System, and 2, control yourself. The former is a self-watering system of styrofoam cells that will last forever and I think of as an essential garden tool. The second, well, the second is the problem with seed-starting in general and tomato-growing in particular. Think on this as you start to sow every last seed in the pack: How many cherry tomatoes can a person eat in one season? (Answer: a plant or two will supply an average family, with several buckets of leftovers.) Sow just a few more of each than you will need, to allow for germination failure, and save the remaining seeds for next year or share with friends. Sow in the next week to have sturdy young plants ready for set-out about Memorial Day to early June.