At the moment, your droplet is going to be accessible only via it's IP address. That may be fine for those that are looking to host a private application. But the majority of users are going to want a domain name they can attach to their server, and promote to others. And for that, you'll need to first register your domain.
Note that you may also reference the DigitalOcean guide for these steps.
If you already have a registered domain name, then great. You can jump straight to the next section. Otherwise, you'll need to register a domain name via your preferred registrar. If you aren't sure who to go through, we recommend Google Domains, they provide a great service, and you would be hard pressed to find a better nameserver provider.
You have the option of configuring your DNS records through your domain provider (if they provide the service), or using Digital Ocean's networking service. If you do decide to use Digital Ocean, then make sure you point your registrars nameservers to those provided by Digital Ocean before adding any DNS records. And don't forget to add the nameservers via Digital Ocean's networking service.
Beyond that, adding DNS records is quite a simple affair. For the most simple setup, you'll only need to add an 'A' type record, which uses '@' as a hostname and redirects to your droplet IP address as the destination. For more complicated setups, including handing subdomains, forwards and other redirects, Google is your friend and should be able to provide context to any scenario you may have.