Advertising, word of mouth etc. It takes time but your big focus needs to be on marketing and how you market the products. Engage in community forums and build your customer base slowly but surely, it'll take a lot of time to become successful but you have to put the work in to it.
Have you tried marketing you tried marketing on forums or related Facebook pages? Many forums have free or paid posting available and there are hundreds of thousands of people looking for good hosting on web hosting and business development related Facebook groups.
The one thing you need to be aware of is that there are thousands of other hosts already. If you're just starting then you'll need to differentiate between you and the others. Good luck!
All the tips above, although globally some opportunities are limited at the moment, but going to a busy area of your city, and talking to shop owners about their web hosting needs also works. Some of the owners of those shops may have been looking for a new web host and not sure where to find one. Some may also be in the need of a new website if it's a service you offer.