Использовать формат строки как
NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:@"1",@"a",@"2",@"b", nil];
NSMutableString *prams = [[NSMutableString alloc] init];
for (id keys in dict) {
[prams appendFormat:@"%@=%@&",keys,[dict objectForKey:keys]];
}
NSString *removeLastChar = [prams substringWithRange:NSMakeRange(0, [prams length]-1)];
NSString *urlString = [NSString stringWithFormat:@"http://mysite.com/server?%@",removeLastChar];
NSLog(@"urlString %@",urlString);