You can harvest leaves after the plants are about 6" tall. Just cut the outer leaves near the ground and don't cut more than about 1/3 of the foilage at one time (the plant needs to be able to make energy to regrow).

The strange plant coming up in the middle of your cilantro is in fact your cilantro. It is likely bolting. My new cilantro seedlings have already started to bolt and they are only about 3 or 4 weeks old. It's because of the warm weather we've had--cilantro likes cool weather.

You can either just keep snipping off the shoot in the center and harvest it as long as you can or plant some more seeds. Most people recommend that you continually plant cilantro every couple of weeks over the summer. Once the cilantro bolts and you let it grow, it will stop producing the other types of leaves that you want to harvest.

Chudak is right. You've left your harvesting a little late. Once it has started bolting (producing the flower stem, flowers and seed at an astonishing rate of speed) there's nothing much you can do to stop it. Harvest the 'normal' leaves and freeze them for future use - those feathery leaves that are soon to form don't taste nearly so nice, although they are edible.

Let the plant go to seed. Gather some of the seed if you are likely to use them, and let some drop to the ground. You'll end up with more plants before too long. Meanwhile, rush out and get some new plants - and this time, harvest as soon as there's enough to be useful, while still leaving about 2/3 of the leaves on the plant. Do the same with the following batch, too. It's a short-lived annual whose life-span can be as short as 2-3 weeks in a hot climate, not much longer in a cooler climate.

The leaves obviously look like tender parsley. Simply cut the leaf and stems off wherever you like. Once the leaves change shape to the longer fern looking leaves, the plants have bolted and they will start to flower and then go to seed. Cilantro is a fast grower and you are only going to get maybe one or two harvests at the early stages. The thinner leaves are Ok too, but the stems now get a bit stringy and tough. Please read through the other very recent posts that are also here and cover the same topic- cilantro