I am a college student living in Belgium coming to visit my parents in the US and I am an American citizen.
Will I need an ESTA?
However, you will need to travel to the US using your US passport.
From the Official ESTA Application website:
Do I need to apply for an ESTA if...?
I am a U.S. Citizen with dual citizenship in a VWP country?
U.S. Citizens are not required to have an ESTA and are required to use their U.S. passport to travel to the U.S.
As a U.S. citizen, you have to travel using your U.S. passport. That does not mean that you should not be taking your Belgian passport as well since otherwise you might get in trouble for the return flight since the airline is not going to like the prospect of letting you into Belgium without any document showing that you are permitted to enter.
If you are a US citizen then no, you will not need an ESTA. US citizens can enter the country without impediment.
You will have to provide evidence of your citizenship. Do you have dual nationality? Or some way to prove you are a US citizen?
If not, you will appear to be Belgian... And will need an ESTA.