There are many, many kinds of petunias available as seedlings in the spring in local garden centers, nurseries and home centers. However, if you wish to have a certain type or color that is not available you can grow them yourself. Be advised that growing seedlings is something of an art and takes some knowledge and equipment to produce sturdy, healthy plants.
Petunia seeds are notoriously slow to germinate, so start them ahead indoors rather outdoors in the garden in order to have seedlings ready to put out in the yard when warm weather arrives. They are also very tiny, so sometimes it is easiest to mix them with some dry sand so that they spread evenly when sprinkled on the growing medium. Six to eight weeks before the date of the last expected frost in your area fill 2 inch deep flat boxes or peat pots with moist seed starter mix or soilless potting mix which are available at garden centers. After sowing the seeds, press down lightly on them to be sure they are in contact with the growing medium. Do not cover them with soil, because they need light to germinate. Water them from below so that they are not disturbed. They will germinate in 5 to 15 days at normal indoor house temperatures. For optimum growth, place seedlings under fluorescent lights for 12 hours daily to simulate late spring light conditions.
See Starting Plants From Seed and see Seed Starting Supplies and Equipment.