The Shabbat candles are lit Friday evening, eighteen minutes before sunset.
Certain communities have the custom to light them somewhat earlier.
The latest one may light the Shabbat candles is sunset; after sunset,
it is forbidden to kindle a flame, and to light a candle would be a desecration of Shabbat.
The earliest the candles may be kindled is plag haminchah, which
is the beginning of the the last eighth of the day (for example, on a day on which there are exactly 12 hours from sunrise to sunset, plag haminchah is 75
minutes before sunset).
For current Shabbat candle lighting times for your location click here