If you grow tomatoes in a greenhouse bed, it's a good idea to plant them a few inches deeper than they were growing in their pots. The stems will put out roots, making the plant more stable, more resistant to drought and able to take up more soil nutrients. Dig a deeper hole than usual and plant so that the soil comes part way up the stem. Alternatively, you can lay the tomato plantlet gently on its side in a trench and gently bend up the stem and tie to a cane. Either of these methods is useful if your young tomatoes have been starved of light and are a bit "leggy". It's still too early to plant tomatoes outside: wait until all risk of frost is past.