Indoor your cannabis grow when you give them 18 hours light per day. If you cut this to 12 hours of light per day they will start flowering.  This means 12 hours of light and 12 hours of absolute darkness. This cycle will have to continue, depending on the kind, for 8 to 9 weeks. After that period the marijuana should be ready for harvesting.. Flowering plants should not be sprayed often as this will promote mold and rot. Keep humidity levels down indoors when flowering, as this is the most delicate time for the plants in this regard.
If you are growing your plants outside you will have to wait until the days grow shorter at the end of the summer/ beginning of the fall. September is the harvest month for weed in your garden.
Look for the white hairs to turn red, orange or brown, and the false seed pods ( you did pull the males, right?) to swell with resins. When most of the pistils have turned color (~80%), the flowers are ripe to harvest. 

Harvesting

Most new growers want to pick early, because they are impatient. That is OK! Just take buds from the middle of the plant or the top. Allow the rest to keep maturing. Often, the tops of the plants will be ripe first. Harvest them and let the rest of the plant continue to ripen. You will notice the lower buds getting bigger and fuzzier as they come into full maturity. With more light available to the bottom portion of the plant now, the plant yields more this way over time, than taking a single harvest.
Do not touch those buds! Touch only the large fan leaves if you want to inspect the buds, as the THC will come off on your fingers and reduce the overall yield if mishandled.

Drying

Do not cure pot in the sun, it reduces potency. Slow cure hanging buds upside down in a ventilated space. That is all that is needed to have great sensi. Drying in a paper bag works too, and may be much more convenient. Bud tastes great when slow dried over the course of a week or three.
 

Use a seal-a-meal to hermetically seal the bag with no air inside. Never ever store your buds when they are not 100% dry.  Freezing your buds is the best way to conserve your weed for a long long time.