Get the signup box and dedicated sales page up now. The sales page should be more salesy vs preselling but you tie that in too. What is it you WANT to offer via your newsletter? From that list, pull bullet points for your "benefits" list on your sales page.
Draft a newsletter or two in Word or other text document so you get a feel for what you want to do. Initially don't send out too frequently; once a month while you are building the first group of pages. Once you have 20 pages, send your first newsletter out, and keep to a regular monthly schedule until you are ready to ramp it up; then bimonthly and eventually weekly newsletters.
All this time you are adding content to your site. Feature new pages in your newsletter, popular news items relating to your niche, and a good juicy bit of content if you can.
Make it worthwhile for others to sign up. Maybe offer a free ebook, tip sheet or something as a freebie if they sign up too.
Just a few ideas