Use LDR with 220 ohm resistor
Unless properly filtered, an LDR has a spectral sensitivity very
different from the human eye, which makes it unsuitable for any kind of
photometric measurement. It is also typically very loosely specified,
so you won't know the illuminance it receives better than a factor two
even if you measure its resistance very accurately. Thus, if you really
need accurate illuminance measurements like, e.g., for complying with
legal requirements, then forget the LDR and get a real luxmeter with a
calibration traceable to the SI lux standard.

Now, if you only want to tell apart substantially different illuminance
levels, like sunny and cloudy, then the LDR is fine. And don't let
Majenko scare you into believing you really need to know the resistance
of your pullup: it doesn't really matter.

That being said, you will get the best sensitivity if your pullup has a
resistance in the same range as the resistance of the LDR in the typical
conditions you will use it. Thus, if your LDR is nominally 10&nbsp;kΩ, I
would avoid pulling up with 220&nbsp;Ω, as the sensitivity would be very
poor. A 10&nbsp;kΩ pullup resistor would be ideal, but the internal
pullup of the I/O pin (20 –&nbsp;50&nbsp;kΩ, 30&nbsp;kΩ typical) should
be fine.

Many tutorials on LDRs use the ADC reading to compute a resistance, for
which you need to know the resistance of the pullup. If you want to
follow this path, just assume a 30&nbsp;kΩ pullup. All your numbers will
be somewhat off, but that makes absolutely no difference, because you
don't know how to relate the resistance to the illuminance anyway.

I would suggest you forget about computing the resistance. Just look at
the raw ADC readings instead. Take some test readings in the typical
conditions that are of interest to you: now you have a baseline to which
you can compare further readings against. Keep in mind, though, that the
ADC readings get lower as the illuminance gets higher. And beware
that the resistance of the pullup, and probably that of the LDR also,
can be sensitive to temperature variations. You may do a hairdryer test
to check whether it gets really bad.

Example code? This one is pretty trivial:

/* Connect an LDR between A0 and GND. */

void setup() {
    pinMode(A0, INPUT_PULLUP);  // enable pullup for the LDR
    Serial.begin(9600);
}

void loop() {
    Serial.println(analogRead(A0));  // print out a reading
    delay(1000);                     // delay for one second
}