Whether you create a beta version of the site first is completely up to you.
It really depends what type of website you're planning to make. Beta's are obviously a good way to gain feedback on your website and its functionality before releasing to everyone. Thus, allowing you to make improvements/fix bugs before everyone uses the site.
In terms of actually getting users for the beta, it's very much a case of marketing your website and its existence well (through social media, advertising etc.), and then providing some kind of 'sign up for the beta' page. You could then close registration for the beta once you have enough users, and devise some method of gaining feedback from users.
I haven't personally created a beta myself, but if I was to do it, I would do the above.
Hope that's of some help.