I just started a curated newsletter about personal finance for millennials. Each issue includes 10 curated articles from various sources about investing, budgeting, paying off loans, and etc. I do not have any subscribers yet.
The easiest way for you to do this, especially that you are targeting millennials, is to use social media advertising linking to a free ebook or webinar about something relevant to your newsletter. The key here though is to require an email opt-in to your newsletter. You'll also have to use some good click bait on your advertising with a title like "Millennials not doing this one SIMPLE thing are missing out on thousands of dollars". You can build a website with an email opt in pretty easy on clickfunnels. You should also start building a brand up on facebook, youtube, and instagram to start getting some organic growth without paying for ads