How can I display my cpu usage (pc) on my arduino?

How can I read out my cpu usage and send it to the arduino?

You need a program running on your PC that collects the data and sends it to the Arduino.

For receiving the data on the Arduino have a look at the examples in Serial Input Basics - simple reliable ways to receive data. There is also a parse example to illustrate how to extract numbers from the received text.

The technique in the 3rd example will be the most reliable.

You can send data in a compatible format with code like this (or the equivalent in any other programming language)

Serial.print('<'); // start marker
Serial.print(','); // comma separator
Serial.println('>'); // end marker

I can’t help with the PC program but I’m sure there will be plenty of advice on PC programming forums.