If you want to cast the spell (crucio) then you must become a Death Eater. Well not must but, if you don't want to be sent to Azakaban for life ten you're better off on his side. Now to become a Death Eater you must do anything the Dark Lord says. (because he'll kill you if you don't) If you cast the spell and are a Death Eater and well...the Dark Lord approves of you then go ahead and cast it, because as it states in the books and movies he breaks out his fellow Death Eaters. Now to the steps in becoming a Death Eater:

1: the easiest way of course is if your parents are already on his side but of course if that was the case you wouldn't have a choice. Draco's a good example.
2: but I don't have it that easy because my parents are in the Order of the Pheonix. but that leads us to the second way of becoming a Death Eater. Leaking information about the Order to the Dark Lord. (that's how I became a Death Eater)
3: just follow him