new grower questions

hey everyone, i'm just wondering if there are any key things i should know? when buying nutrients for my plants.
I know for growth i need food high in nitrogen, and for flower i need one high in phosphorus and potassium. but is there anything else i should know?

Thanks.

So these are the seedling i know its early to feed, i'm not going to i just wanna have everything ready when its time to.


out.