Is it hard to transfer contacts from one iPhone to another iPhone? I just bought a new iPhone 6s, and want to move all the contacts from the old one to the new one. Can iTunes or iCloud help me do this, or are there any other ways?
Yes either iCloud or iTunes can do this.

The overview is that you take a backup from the old iPhone and restore during setup on the new iPhone.

Apple has a support page for this: Transfer content from your previous iOS device to your new iPhone, iPad, or iPod touch