When preparing your car for a road trip the most important thing is to ensure that your vehicle is safe. Check the tires including the spare to verify that they are properly inflated, have good tread, and aren't damaged in any way (no gouges or protruding belts) and test the jack. The brakes should be checked to make sure that the pads and rotors are thick and have plenty of life in them. The suspension system should be inspected to verify that the shocks aren't worn out and that none of the springs are broken. Fill the washer reservoir and test the windshield wipers to make sure that they do a good job of cleaning the windshield. With a helper make sure that all the lights are functioning correctly.
You should also have a fully stocked first aid kit and fire extinguisher close at hand. Once you have everything ready to pack make sure that you are not overloading the vehicle. The maximum passenger and cargo weight your vehicle can carry should be specified in the owner's manual or on the tire information sticker on a door jam.