The comments should have put you on the right path. You are at the point we all began with serial communications.
PuTTy is very useful, keep it handy. Use it to test your hardware (because you know IT works) and whenever you run up against a problem come back to it to do basic testing.
This is a very basic tutorial to get you started:
http://csharp.simpleserial.com/[
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As you can see in C# we always come back to the SerialPort class:
https://msdn.microsoft.com/en-us/library/system.io.ports.serialport(v=vs.110).aspx[
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There are many excellent articles here on CP but I will pick a couple by experts. Start with this which has a very good background explanation:
Serial Comms in C# for Beginners[
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and this one:
Arduino, C#, and Serial Interface[
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For Arduino substitute your own device.
This will get you started but as always come back here with specific questions.