Do you know how long it's been growing? I don't think you're supposed to harvest unless it's been in the ground for around 2yrs.
Anyway, harvest when it's about 6" tall, cut off the shoots with a sharp knife about 2cm below the surface of the soil. Asparagus can grow very quickly and in ideal conditions can grow abouy 6" in a day. You might be looking at harvesting daily for about 7-8 weeks, then stop. The remaining shoots will enable the asparagus plant to build up the roots.
Pretty much what ShutMeUp said. I'm not sure about letting it grow for two years, but my garden has had asparagus for many years so it was never a factor for me. But yeah, just get a knife and cut it down, nothing more to it.