The first 6 weeks the plants need 18 hours of light per day. After all it is summer for them. This is called the growing phase. After 6 weeks you will only give the weed plants 12 hours of light per day. Because of this the plants will think the days are becoming shorter and this means they will flower. So this is the so-called flowering phase.
Best for the plants are natrium lamps which are used by real growers. However this is not absolutely necessary. It can also be done under neon lights or normal lamps combined with daylight. However it is wise to use a type already coming from a temperate climate, for example Northern Lights or a Skunk.