The answer is: it depends. Just like a basketball strategy depends on your strengths and weaknesses, so does your marketing plan.
Do you have a big budget? Go big on retargeting, SEM, and social ads. Hire a PR firm, preferably one that has tangible experience launching apps similar to yours. They will already have the contacts they need to do it right.
Do you have time? Find online influencers who would be reputable reviewers of your product. Build relationships with them. Ask them to review your product, and write content that they would feel happy to promote. Build "a community". Go to events and speak.
If you do not have time or money, you can pray. There really is no magic bullet here. Even Facebook put boots on the ground at campuses in the early days.
Creating a solid marketing plan given what resources you have is your first start. Using a tool like Trello or ProjectPlace to break down your strategy into actionable items that you periodically assess is a good way to make sure what you say actually gets done.
