Accepted custom is the woman accepts shabbos at the time of lighting - when she says the bracha. Others may continue to do work until just before sundown. 

When a man lights, his Shabbos has not yet commenced, it's either a few mins before sunset or when shul he's in does last part of lecha dodi.

3 reasons for candle lighting:

honor, enjoyment, peace in the home.

How?

Usually one makes a blessing prior to doing the mitzvah. When a female lights Shabbos candles she can't make the blessing first, as then Shabbos would begin before she could light the candles! So, she first lights them and then makes the blessing.

If a man is lighting he first makes the blessing as for a man Shabbos doesn't come in with candle lighting but with sunset/last bit of lecha dodi.

What?

Candles or olive oil. Olive oil is preferable.

Electric lights - are acceptable when no other alternative but shouldn't make a blessing on them. e.g. hospital.

When? 

Custom - 18-20 minutes before sundown. No later than 2 minutes b4 sunset. As early as "plag hamincha" which is 1 and quarter halachik hours before sunset.

How many?

Shamor and Zachor - the 2 aspects of Shabbat. 

Basic requirement - 1 candle. Some have custom of adding on for each child.

If didn't light:

If negligent, doesn't apply to emergencies, has to light an extra candle forever!

What if I'm running late? the mitzvah is not to light.

When does "early shabbos" begin? 

If 1 shul in town then it is binding on the whole community but if more than one shul it does not apply. When it is for convenience it is independent for each member of the household, i.e. if the husband accepts it early it doesn't impose on the wife.

Please see the beautiful blessing one can say at candle lighting time.

Also not myzmanim.org has an app that you can see any Friday the earliest time for lighting called plag hamincha and the latest, which is just before sundown.