Everyone else already included the basics, such as an opt-in page and traffic, but I'd like to emphasize this:
If you can't keep people on your list then you haven't built a list. You need to know how to keep your subscribers engaged with your emails and get them to sign up for the offers that you're promoting.
This is not an easy task for the majority of newbie marketers, especially those that struggle with writing.
If you don't know how to write sales copy and have no experience in writing marketing emails, then I would suggest that you either use email marketing PLR or you outsource the job to a writer.
Almost anybody can get signups but if you take a closer look, the marketers who are KILLING IT are the ones that really know how to sell to their subscribers with high quality content.