1. I was told that you are not supposed to eat the first
harvest..... Is this correct ?
I've never heard of that, and I have always eaten all the stalks
I've picked.

2. When should I harvest the plant ?
You can pull stalks as soon as you like. I pulled my first ones
this year two or three weeks ago.

Just don't pull them all at once - take a few and eat (or
freeze) them, then a few more and so on. They are sweet and
tender in spring, and they get tougher as the season goes on,
but if you always choose fresh young stalks they're always good.

If you see stalks with flower heads developing, cut them out and
throw them away to prevent the plant bolting, as it won't put
out fresh stalks once it's done that.