How do I save an NSString as a .txt file on my apps local documents directory?

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); 
NSString *documentsDirectory = [paths objectAtIndex:0]; // Get documents directory

NSError *error;
BOOL succeed = [myString writeToFile:[documentsDirectory stringByAppendingPathComponent:@"myfile.txt"]
      atomically:YES encoding:NSUTF8StringEncoding error:&error];
if (!succeed){
    // Handle error here
}

Something like this:

NSString *homeDirectory;
homeDirectory = NSHomeDirectory(); // Get app's home directory - you could check for a folder here too.
BOOL isWriteable = [[NSFileManager defaultManager] isWritableFileAtPath: homeDirectory]; //Check file path is writealbe
// You can now add a file name to your path and the create the initial empty file

[[NSFileManager defaultManager] createFileAtPath:newFilePath contents:nil attributes:nil];

// Then as a you have an NSString you could simple use the writeFile: method
NSString *yourStringOfData;
[yourStringOfData writeToFile: newFilePath atomically: YES];

you could use NSUserDefaults

Saving:

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
[prefs setObject:@"TextToSave" forKey:@"keyToLookupString"];

Reading:

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
NSString *myString = [prefs stringForKey:@"keyToLookupString"];