If you are using Cloudflare you can certainly use their product to create firewall rules which will help to secure your droplet.

However, cloudflare is operating at the DNS layer and then routing requests to your droplet which you will want to secure independently and here are some resources to help you get started:

Using Managed Firewalls
https://www.digitalocean.com/docs/networking/firewalls/how-to/configure-rules/

Manually
https://www.digitalocean.com/community/tutorials/how-to-set-up-a-firewall-with-ufw-on-ubuntu-16-04

You do not want to limit outbound traffic to just Cloudflare as that will actually break your website. Cloudflare just handles the inbound request through DNS and then routes it to your droplet, after that your droplet is communicating directly with the user that created the request. So if you limited outbound traffic to just Cloudflare it would break connectivity for your domains.