First time grower

Plant is about a foot and a half tall, when should I start flowering stage and how??
I'm using the organic grow method and I am growing in my room

How old is it? You start the flowering stage by adjusting your veg light cycle from 18/6 to 12/12. Wait for the plant to show sex before you flower it if you are new. if you have only one plant then id say keep it male or female and get some dank buds or get a shit load of seeds to grow off of.

It's about 3 weeks old and how do I determine sex?

You have to switch the light to 12 on and 12 off to see the sex. It will have white hairs if female. If male there will be green balls. Not seeds. The balls are pollen sacks. Your plant should have 4 to 6 nodes b 4 u switch the lighting

you do not have to switch to 12/12 to see sex. The plant will put out preflowers around week 6 even while under 18/6

the height of your space should help determine when to flower plants stretch quite extensively at times during the first couple of weeks of true flower. If you run out of space tie some branches down.