NSString *myString; //Assuume the string you want to write is this
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *path = [documentsDirectory stringByAppendingPathComponent:@"myfile.txt"];
[myString writeToFile:path atomically:YES];

Write string to file in documents folder

Objective C