There are many approaches as to when to start Shabbat:

Men
For men, whichever is first:

• At sunset, or
• When they light candles intending to begin Shabbat then, or
• When they say Mizmor shir l'yom haShabbat.

Women
For women, whichever is first:

• When they light candles intending to begin Shabbat then (most people light 18 minutes before sunset but local customs can vary; e.g., Jerusalem), or
• In case of urgent need, just before sunset if they have not lit candles.

 
Note Even if a husband has finished ma'ariv for Shabbat, his wife is not required to start Shabbat when he does, and she may still light her candles at the normal candle lighting time. The husband does not need to wait outside until she has lit. However, the ideal situation is for the home to be ready (including table set) by the time the husband has finished ma'ariv and has returned home from synagogue.
 
Community-Wide Considerations

• If an entire community begins Shabbat at any time earlier than sunset on Friday, EVERYONE must begin Shabbat at that time.
   
• If there are at least two minyans in any community, no one is required to start Shabbat with the earliest one (but if you associate yourself with one of those minyans, you must follow their custom).