Watermelon, like other cucurbits, like a well drained soil rich in organic matter. Plenty of manures, compost, blood and bone etc. The richer the soil the better. They like a fair bit of water while they're growing but can take care of themselves once established so long as they get a deep watering weekly or so. Mulching helps and keeps the fruit clean as well.
Plant out once the soil has warmed to over 12-15 deg c and the seedlings have their first set of true leaves. Water before and after transplanting with a weak seaweed fert to reduce transplanting shock. Pick the fruit when the little 'pigs tail' opposite where the stalk joins the vine dries out.