If you download and install ST's Cube etc you will end up with (amongst
a LOT of other stuff) the library code that you could use if you want
I don't use these - they try to be universal but end up being hard to
understand and slow.
To drive the ADC directly you will need to study the reference manual
and register descriptions and it may well help to look at some ST
examples (from the Cube again).
To address your specific problem:
Assuming a single conversion of one channel and starting after a
write the channel to be converted into the SQ1 field of ADC_SQR3
make sure the L field of ADC_SQR1 = 0 so the ADC does just one conversion
set the SMP field for the channel in question to the sampling time you
want (ADC_SMPR1 or ADC_SMPR2)
enable the ADC by setting bit 0 in ADC_CR1
start the conversion by setting bit 30 in ADC_CR2
(All the above based on Ref manual for STM446xx - check details for your
You will need to enable the ADC clock but you must have got there already.