The plants have probably been grown in a greenhouse or hoop house, and so they will need to acclimatize to outdoor conditions before you plant them. You can leave them in their pots. Put the plants outside in a sheltered place, not in direct sunlight at first, and then over a couple of days gradually give them more sun. Give them water either early in the morning or in the evening. Bring them inside at night if there is a risk of frost. After a few days, they should be "hardened-off" and you can plant them in the garden.