There are essentially 3 methods
1 Method requires you to have money(at least $5) on paypal
1 Method requires you to go to the apple store
1 Method is sort of like method 1 but you dont need to go to the apple store and can just use an app
2 Of the methods are essentially "free", the paypal one requires $5 so you need to start with $5, but you dont lose it, so technically its "Free"
2 of the 3 methods are bulletproof which means you cannot get in trouble. This is a good guide, it provides 3 very basic, down to earth concepts to get free NX.