Well, you should give the server a static internal ip-adres:
Edit /etc/network/interfaces (see the iface eth0 part)
sudo nano /etc/network/interfaces
so that it looks like:
iface eth0 inet static
address 192.168.1.23 # Your ip address here
netmask 255.255.255.224 # Your netmask here
gateway 192.168.1.1 # Your gate address here
dns-nameservers 18.104.22.168 22.214.171.124 # Google's nameservers
After that either restart or do the following:
sudo ifdown eth0
sudo ifup eth0
MAKE SURE you don't use an already used internal ip, so just make it something like 192.168.1.200.
Now you log in to your modem, and add a virtual server, let it point to the new internal ip you set for the server. Open the correct ports and you should be ready to go.
If your modem doesn't support a virtual server, just open the required ports.
The default website port is 80. But at dot.tk you can always say, refer to external-ip:port.
I hope this helped, and you can always ask when this isn't clear enough(since its a wall of text).
Have a nice day