You need high-quality backlinks from other sites in your niche. What's a high-quality backlink according to Google? A link from a page with unique and high quality content on a domain with high authority. As already mentioned, it will cost you. You can try exchanging link with other sites but unless you have a high DA, you won't find anything worthwhile. If you're willing to spend, just make sure you don't get like 100 at once. Spread em out evenly over time.