If buying seedlings in a pot, plant them deep enough so half the plant lies underground. Tomatoes develop roots along their stems and they will grow into stronger plants.
find an angle
If your seedlings are leggy, dig a shallow hole and lay the plant in it sideways. It will quickly straighten up as it searches out the sun.
cut to be kind
Remove shoots that are growing between the main branches. They take away energy from the plant that will be needed to produce the fruit.