You finally pulled the trigger and rented your own Project Zomboid server from Indifferent Broccoli, and you have no players. The biggest roleplay servers are so far ahead of you, and you have a long way to go. Luckily, it's easy to promote your server around the internet, here are some places to get started.
When the first players start to join your server from the server browser, they'll want to have a good experience and feel like they have something worth coming back to. This means to start out, you may want to lower the zombie density and difficulty of the game, so the first players can get establish quickly and want to come back to their progress. The more progress players can make off the bat, the more likely they are to come back.
The best way to get new players interested in your server. You can promote your server in Indifferent Broccoli's promotion channel, as well as the promotional channel in the Official Project Zomboid Discord server. There are many active Project Zomboid players in both servers who are looking for a new home. Make sure to post pictures of your server or any art you have made, as well as your server description.
Make sure to explain in your server description what settings are enabled, if there are any wipes, the difficulty, and the type of server you're trying to host whether it be roleplay or just regular Project Zomboid. Make sure to mention any large scale mods such as big map mods, which can draw players in if they've never explored those maps.
Create a Discord server for your server if you haven't already, and host a prize draw game that players can join each week. The prizes could be special weapons or loot that will set them apart from other players. If players know they can come back to win a prize each week, it will keep them committed. Hosting prize draw only items will make sure that players who win will continue to come back as they will have rare items that no one else has.