@Ngc1514,
FB doesn't do such things terribly well. And you might want a group rather than a page (I'll explain why below).
Here's my understanding of what you can do.
1) To make the group open to all for
viewing, it's got to be either an Open or a Closed group. The main difference between the two is that anyone can join an Open group, whereas people in a Closed group need approval from someone in the group (I don't think the approval has to come from a moderator or admin)
2) To restrict posting, you could restrict it only to people with mod or admin status. That could work for a very small group.
3) For a larger group, that's tricky. Having a group of 20,000 people who are all at moderator level or above is probably not going to work. In part this is because mods have more privileges than just posting. You could get into some trouble if anyone turns out to be untrustworthy (mods can't outright delete a page or group, but they can cause mischief all the same)
4) You can also require posts to be approved by mods or admins. That's a standard for pages. I can't recall if that's a setting for groups
So - bottom line (as I said above) - FB is bad at such things. Decide what matters the most to you, and work from there.