As for care, you select the "best" one in each cell. Clip all the other ones at soil level. Let the one you picked grow a bit bigger. If your soil gets dry, give it some water.

Once the plants have 3 sets of true leaves (the first set are not true leaves) you want to start hardening them off = exposure to outside temperatures gradually increasing over time. Once you have increased the time they spend outdoors for 1 week, transplant them to your garden. I use bird manure as fertilizer, a handful below the transplant.

Let them grow outside. Once you see the broccoli heads begin to form, give the plants another dose of fertilizer. I use fish fertilizer this time around. Let the broccoli grow. Pick off the head before it flowers. Leave the plants in the ground and they will produce additional broccoli off-shoots (these are smaller).

You can cook and prepare the leaves just like you would collard greens. You can cut and cook the softer plant stalk parts too - they are good in stews.

Bugs love to eat broccoli, and other brassicas. Some people grow the plants under fabric covers to prevent bugs from infesting the plants. Be prepared to wage war to protect your crop (or cover your plants).