veg know how

when and how can i veg my 5 week old marijuana plant?

Huh? If it's growing and not flowering, you are vegging your plant.

I think you mean flowering. The common wisdom seems to be that it's based on height although some might want to look for preflowers and node development. The plant should be atleast 12" high, some would argue higher. Your plants, depending upon strain, will grow to about two to three times their size during the flowering cycle. You need enough room for them to grow to this height. You should post some pics of your plants so people could judge if they're ready for flowering.