Not a member yet? You'll have to sign up for an account before you'll be allowed to post in the forums.
Isn't that obvious?
You might want to tweak the "you'll be allowed" phrase to be "you are allowed" or "you will be permitted". Not sure how well translators handle our contractions.
You could also add that visitors are able to freely browse and search the forums without the sign up requirement.
And since signing up requires an email address you should make clear that the email address will not be sold or distributed to any business partnerships (such as WhenYou) nor used to send SPAM in any way shape or form. So many unscrupulous sites scam users into registering specifically for that reason that it has made experienced users gun shy about signing up for things.
And Happy New Years!!