You need to be a lot more forthcoming about what you are doing, exactly what HC-06 device you are using, and post the code, using the hash icon.
I bought a bluetooth module HC-06. I connect Vcc to 3.3v,
This is OK if, and only if, you are using the bare HC-06 module
tx to rx and rx to tx.
Yes, hardware serial D0,D1
in some tutorial on youtube someone uses two resistors
Good idea for Arduino Tx
then I change the serial port from 24 (the usb one) to 35 (the buetooth one). but the in the serial monitor nothing is printed. what is wrong?
This is unintelligible but it may be that the only thing wrong is that you are trying to use the serial monitor as a terminal. Make sure the USB cable is disconnected and use a proper terminal programme like RealTerm through a properly defined COM port.
You might find the following background notes usefulhttp://homepages.ihug.com.au/~npyner/Arduino/GUIDE_2BT.pdfhttp://homepages.ihug.com.au/~npyner/Arduino/BT_2_WAY.ino