There are a number of ways:
1. SEO (search engine optimization)
2. PPC (pay per click advertising)
3. Referrals (attracting direct traffic from a link on another website)
This is a huge question, but those three ways are the primary ways of going about it. You should start by sorting out the on-page SEO of the website, either yourself by trying to teach yourself, or paying a reputable SEO company to do so. You can work from there!