How can I completely secure my system/network ?
Switch off your system(s), unplug them, put them in a titanium lined vault burried in a concrete bunker, surrounded by nerve gas and well paid armed guards.... Even then you shouldn't stake your life on it, there is still a slight chance someone might break in. So, to answer your question there is no way to completely secure a system.
Still, a multi-layered approach to properly securing your network can greatly reduce the risks of someone breaking in. Keeping up to date with security fixes and updates for your OS, combinations of software/hardware firewalls, anti-virus/anti-trojan/anti-spyware software, as well as keeping well informed of vulnerabilities and potential exploits all contribute to better securing your system/network.
Most importantly, don't feel a false sense of security just because you run xyz firewall and fully updated anti-virus program, keep in mind there is no 100% safe system !