Good bright light is a compulsory requirement for a healthy Croton plant and this cannot be ignored. They will cope with some fairly low light conditions, but after a few months of this, you will notice that the leaf colors and markings have massively diminished or gone completely dull.
Some direct sun falling on the leaves is ideal, especially that from early mornings and late afternoons. But avoid prolonged intense midday sunlight to prevent leaf scorching.
The second most important Croton care requirement is watering. Too little and the leaves will fall and too much will cause a similar effect. You need the knack (and some practice) to get the balance right.
So how do you water a Croton correctly? During growth, which tends to happen when conditions are warm, you want the soil to be evenly moist for the majority of the time. That's moist, not soggy, wet or saturated, and equally as important, not dry.
If you can pour water out of the pot, or there is wetness in the drip tray an hour after watering you're overdoing it. If you're letting the soil dry out completely before you water again, you 're making the opposite mistake of not giving enough.
In Winter you still need to water your plant, but much less often. It's best practice at this time of the year to let the top inch or so to dry out fully between waterings. Doing this will prevent accidental overwatering.