One of the easiest mods to do will also give you the most gain in HP excluding power adders like nitrous and blowers. There is a large number of companies that make aftermarket air intakes for your truck. Examples of these companies is K&N and Airaid. Some of the air intakes (like the Airaid) only replace the factory intake system from the MAF (Mass airflow sensor) forward while others replace the whole intake system. These kits include a tube that runs from your throttle body to your MAF. Most all the kits have a heat shield box. This heat shield is made out of either plastic of metal and surrounds the filter to block heat off of the motor from being pulled into the intake system. These kits will not void your warranty. Below is pictures of some air intake systems for your truck.

Airraid Intake K&N Gen 2 K&N Gen 2