I need to set up a computer using static IP such that I can acces it from outside with a particular software. Of course, this will expose my computer to hackers' world and a firewall is a must (may be something more).

The host computer can be set aside for this purpose only so I don't have to allow other access. How can I ensure that the host computer will block any unauthorised access except mine via that software? I have no idea how this SW works, e.g. what port it calls etc. Is there any utilities I can install in the host/remote computers such that I will know what port is active or used? What firewall (preferrably freeware) is the best choice.

Thanks for your advice.