1 Install firewall

apt-get install ufw

2 Change ssh port

nano /etc/ssh/sshd_config

in there find #Port=22
change it to: Port=yourwishedport e.g. Port=5050, the default port is 22, so you will need to specify the new connection port in your ssh like this: ssh username@hostname -p <port>

systemctl restart ssh
systemctl restart sshd

sudo ufw default deny incoming
sudo ufw default allow outgoing

sudo ufw allow ssh

sudo ufw allow 22

sudo ufw allow <sshPort>

sudo ufw allow 3000:3100/tcp
sudo ufw allow 3000:3100/udp

sudo ufw enable

sudo ufw allow from 203.0.113.103

sudo ufw allow from 203.0.113.103 proto tcp to any port 22

Further examples and more detailled explanations: digital ocean blog