There are a number of NSString class methods that create and return strings (refer to the NSString Class Reference documentation for a complete list). The one you're likely to use most often is stringWithFormat:

NSString *nameString = @"Fnord";
NSString *newString = [NSString stringWithFormat:@"The real %@ is %d", nameString, 23];
       // newString is now @"The real Fnord is 23"

Common string format specifiers are:

See the full list of String Format Specifiers in the String Programming Guide for Cocoa.

The length of a string can be obtained with the length method:

NSString *nameString = @"Fnord";
int nameLength = [nameString length];       // nameLength is 5