Unfortunately there isn't anything you can do to completely stop DDoS attacks. You can start by whitelisting your server to trusted IPs (see https://www.digitalocean.com/community/articles/how-to-setup-a-firewall-with-ufw-on-an-ubuntu-and-debian-cloud-server).

Yea, I need this too, my server also keeps getting DDoSed. And DigitalOcean keeps suspending my droplet's network.

Start with easy ways to limit connections and access. I would start by lloking into TCP Wrappers, xinetd, and iptables. These wont help against any sort real DoS but you should be able to lock out random ne'er do wells portscanning the universe.

In a way its kinda like you have a good hosting but it goes down really easy. 1 year ago a friend of mine was at digitalocean and got constant ddos but the network to his droplet would never get suspended.... now? even if its just for 1 second ddos it goes down..... please use an anti-ddos system like OVH when they get an attack the server redirects the ddos to the mitigation center.