if i use potting soil perlite and that other stuff on that mix in the video when should i have to fertilize or how much i have no idea on that..
Dude u really need to start reading ok, there are a few basics that u need to know in order to understand the needs of a MJ plant...
Now answering ur question, if u choose to have ur plants in soil when u get the soil at ur garden center growshop or whatever (dont use soil from ur backyard) u will be able to read in the soil bag the nutritional value of the soil.At this point let us just focus on MACRO NUTRIENTS, these are the most important in plant development, and somewhere in soil bag u can read something like this:
N-P-K -> 20-20-20 or 20-6-10
These stand for N - Nitrogen P - Phosphorus K - Potassium
So what u are looking for at this stage (seedling stage and vegetative cycle) is a high value of (N).Nitrogen is the Macro nutrient wich Cannabis uses more at this stage, altho it also uses (P) and (K)...just like the ratios i mentioned above...
This will mean that u wont need to feed nutrients to ur plants for the 1st 3 or 4 weeks...(if u check the vid u watched the grower says exactky this)...I hope this will get u started altho i know if u are serious about growing more questions will pop in the near future.
And remember READ READ READ READ and then READ some more....Peace and Pot