I would top the plant a few weeks after germination (basically any time in the veg period is okay to top). Then I would let it veg for a little longer, just so it grows a bit more. Maybe 2 weeks would be nice, but ultimately it's your decision!
when top u cut off some. u know why not just throw weed away. idk everyone has there own system of doin things. but bottom line it's a weed. it's pretty tough. and if ur a newbie try lst toping and leaving one alone altogether. try it all experimenting is half the fun