What part to use from fresh Basil leaves? I was wondering what am I supposed to cut from basil leaves, do I only use the “blade” until the petiole or I should cut that too?

I know some things must be chopped in the right places because it may add sourness. 

Also, can they be boiled or they should be eaten “raw”?
You can roll the leaves and thinly slice them (chiffonade), that's my preferred method. I've never had an instance where I cut basil and made it sour though.

I've never heard of boiling them, but I assume you could get the essential oils out with that method and could make an infused water. You can fry the basil leaves in oil as well to make a crunchy topping!