Seeds vs Plants

For most things, you can plant just seeds. For example, these do well from seed:

Cucumbers
Melons
Squash
Pumpkins
Corn
Spinach
Beets
Beans
Swiss Chard

These are almost always started as transplants:

Tomatoes
Peppers
Eggplants
Celery

These are good to start indoors to plant in a spring garden when germination is poor:
Broccoli
Cauliflower
Cabbage
Lettuce
Peas

You could plant these at the same time you plant your spring garden and then transplant if the ones you plant outside don't come up, or start them about March 1. You could also start successive plantings of lettuce inside March 1, March 15, April 1 and April 15.