HEllo thanks for your question !
In order to use internal ADC you must give the reference voltage to the microcontroller . Connect your AVCC to reference voltage(5 volt generally ) and AREF pin to Ground . So that internal ADC of microcontroller take suitable reference . I have made a nice tutorial for you related to how to use ADC of microcontroller in Atmega32 microcontroller please visit this reference link https:www.electronify.org
Thank you 
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