It depends upon where you are located. Rhubarb is generally one of the first food plants to be harvested. You should be able to harvest the plants in mid to late spring. It could be as early as April or May but the plants will last until September. If you are in the northern United States there are usually two harvest times, one in late April or early may and the second in June-July.
You will want to use the stalk portion of the plant for most rhubarb recipes. Leaves can be cut and removed.