Analog read will return a number in the 0 - 1023 range for an input voltage in the 0 - 5V range.
You have to come up with a mapping scheme.
But if your synth is the typical 1 Volt / octave it shouldnt be too difficult to do the math.
Another thing you should take in to consideration is that the readings you get from analogRead is probably not going to be rock steady, so they might change a bit from reading to reading even if you do not cahnge the CV. You can deal with that by taking a bunch of readings and then calculate the average, before you convert it to a MIDI note value.