Welcome to TestComplete world!:)
"I am a beginner in both TestComplete and Scripting (Java). "
"'Cannot read property 'SerialPort' of undefined."
Did You try to do steps from the documentation? You can find there this thread.
System.IO.Portsnamespace is included in the system.dll assembly. This assembly is registered in the Global Assembly Cache (GAC). In order to use the namespace in scripts, register this assembly in the CLR Bridge Options page of the Project Properties editor:"
Let me know if this helps You.