Hi! This is my first weed plant and I don’t kno...

There a few rules you can follow. First, as soon as you see pre flowers (the little hairs on the base of the nodes) then the plant is sexually mature and can go into flower stage (doesn’t mean that you MUST switch to 12/12). Second, when your plant reaches half the height you want (or have available) then you switch, since most strains will double their height in flower stage (sativas can triple their height). If you want to delay the flowering, then you could train your plant so it bushes out instead of growing up; try TOPing, LST and / or super cropping. Hope that helps.