problem with sending data

Hi, I am developing a project that is based on the serial communication of arduino with an interface in labview, the detail is that I can not convert the data (decimal number) into an analogue number, since I need to vary the amplitude of an electrical resistance for water heating, I would appreciate your help

I need to vary the amplitude of an electrical resistance for water heating,

How are you going to do that? The Arduino is not a programmable potentiometer.

You can use the Arduino with a digital potentiometer such as these digital potentiometer - Google Search. This will be fine for controlling a circuit but...

... they are not for carrying any significant current/voltage. So if you want to control the power to a heating element you will have to have something rather more robust. Google will help with lots of options you can build or buy depending upon your requirements.