This page has the numbers for a tomato crop: 24 inches between plants with 36 inches between rows, seed started six weeks before your last frost and set out a week after it, an inch and a half of water a week, soil at pH 6.0 to 6.8, and 8 to 15 pounds of fruit per plant across a 60 to 85 day season. The calculators above apply those to your own bed and frost date.
Three things account for most disappointing tomato crops, and all three are covered below: planting out into cold soil, watering that swings between soaked and dry, and spacing that leaves no air between plants.
Starting them off
Sow indoors about six weeks before your last frost date, in a tray on a windowsill or under a light. Earlier is not better: a tomato sown ten weeks out becomes tall, pale and root-bound long before it can go outside, and never fully recovers from the check. Seedlings want warmth to germinate and then bright light to stay stocky. If they lean and stretch, they need more light rather than more heat.
Pot them on once they have their first true leaves. Tomatoes root from buried stem, so each time you move one up you can set it deeper, which produces a sturdier plant.
Planting out
Wait until a week or two after your last frost date, and until nights are reliably above 50F. Tomatoes set out into cold soil sit and sulk, and a plant put out two weeks later often overtakes one planted early. Harden them off over a week of increasing time outdoors first.
Plant deep — deeper than looks right. Bury the bottom third of the stem, pinching off the leaves that would be underground, and the buried section will root along its whole length. Leggy seedlings can be laid almost sideways in a shallow trench with just the top rosette above ground.
Space them 24 inches apart with 36 inches between rows. That looks absurdly generous in May and about right in August. Crowded tomatoes get blight and mildew because air cannot move between them.
Put the support in at planting time, not later. A stake, cage or string driven in beside a six-inch seedling is easy; the same job around a four-foot plant means broken stems.
Water and food
Tomatoes want deep, consistent watering — roughly an inch and a half a week, more in heat, delivered in two or three long soakings rather than a daily sprinkle. Consistency matters more than quantity: the swings between drought and flood are what cause blossom end rot and split skins. Mulch two or three inches deep once the soil has warmed, which evens out the moisture without any further effort.
Water the soil, not the plant. Wet foliage is how early blight and septoria get established, and both are much easier to avoid than to cure.
On feeding, go easy on nitrogen. Rich nitrogen gives you a magnificent green plant with little fruit on it. Work compost in before planting, then switch to something lower in nitrogen and higher in potassium once the first fruits set. A soil test is worth more than guesswork here, particularly for phosphate, which accumulates in long-cultivated beds.
Determinate or indeterminate
Worth knowing before you buy, because the two behave completely differently.
Determinate varieties grow to a set size, flower, and ripen most of their crop within a few weeks. Good for sauce, good for containers, and they need little or no pruning.
Indeterminate varieties keep growing and fruiting until frost stops them. They need real support and benefit from having their side shoots removed. Most heirlooms are indeterminate.
Picking
Pick when the colour has developed and the fruit gives very slightly to a gentle squeeze. A tomato will finish ripening off the plant, so in a wet spell it is better to pick a nearly-ripe fruit than to lose it to splitting. At the end of the season, green fruit brought indoors will ripen slowly in a paper bag, or can be used green.
Do not refrigerate a tomato you intend to eat fresh. Cold permanently flattens the flavour.