I feel like I've answered this question many times before, but the alleged duplicate recommends using NSUserDefaults and I think that's poor advice for saving the data that your app manages. (The dupe is from back in 2008, BTW; the iOS landscape was somewhat different back then, but even so I still don't think the most popular answer gives very good advice.)
Since you're looking at having your users log in, there may be a server that they're logging into. If so, saving the data on the server is the right thing to do. Exactly how that work will depend on your infrastructure, but you'll probably GET and POST data to a web server using HTTP. There's a LOT of information on the net about how to go about that (lack of research is probably why you're getting down votes), but you'll either use NSURLConnection and NSURLRequest directly, or you'll use a third party library like AFNetworking that makes it even easier to access a web server.