You can always use social media of course, as many people above suggest. However, for some kinds of websites, the target audience may not be on those social media. Your target audience may prefer getting directed to your site using search engines. In that case, you will have to hire someone specialised in SEO. If you want to give your visitors an extra boost, and depending on the content of your site, you can also buy advertising space (at a particular website concerning a particular subject for example) to redirect more people to your site. That doesn't have to be expensive, if you tackle it well.