From the time one accepts tosefet Shabbat, he must refrain from all melakhot forbidden by Torah law. Rabbinically prohibited actions are also forbidden at this point, unless the melakha to be done is for a mitzva, for a Shabbat need, or for some other great need, because in such situations the Sages did not intend for their prohibitions to apply. For example, if one forgot to tithe produce and wishes to eat it on Shabbat, he may tithe it even if he has already accepted Shabbat, because tithing produce on Shabbat is only prohibited rabbinically.
One who has already accepted tosefet Shabbat may ask a fellow Jew who has yet to accept Shabbat to do melakha on his behalf. Women generally light candles and accept Shabbat at the time listed on calendars. Men, in contrast, go to shul to pray the weekday Minĥa (which usually begins shortly after candle lighting), and only then accept Shabbat. In the intervening period, a woman may ask her husband to do melakhot such as turning on a light or adjusting the oven, even though she has already accepted Shabbat (SA 263:17; MB ad loc. 64). Similarly, on Saturday night one who has not yet made havdala may ask one who has to do melakha for him.
A woman who has good reason to travel by car after lighting candles, such as to go to the Kotel, to shul, or to see her family, may get into a car if the driver has not accepted Shabbat yet. However, she must make sure not to open or shut the car door herself if this would make the light inside the car go on or off (see Harĥavot).