The bad news is that you shouldn't harvest asparagus until its third season in your garden.
Once your asparagus is established, here's how to harvest it: when the spears are six to ten inches above the soil line, but the buds have not yet opened, they are ready to harvest. Simply cut or snap the spear off at the soil line. You can harvest asparagus for six to eight weeks each spring. If you are noticing decreased production and vigor in your asparagus plants, stop harvesting and let the plant store energy for next season.