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.
There are also some things to check before a road trip to help ensure that your car is reliable and that you get to your destination. The first is to check the vehicle's maintenance schedule and perform any upcoming routine maintenance such as an oil change or transmission service. You should also check the oil, coolant, transmission, differential, and power steering fluid levels. Ensure that the air filter is clean, and check the belts and hoses to ensure that they are in good shape. Test the battery to make sure that it has plenty of juice.
To help make the trip comfortable and a bit less stressful make sure that you carry a spare key or two so that you don't get locked out of your car. Also ensure that you have the vehicle's ownership and proof of insurance. Packing snacks and drinks will keep you comfortable and prevent you from having to pull over whenever you get hungry or thirsty. Games or videos can help keep passengers distracted and make the time pass more quickly. Toilet paper or tissue paper can come in handy to clean up a mess, and baby wipes or hand cleaner can remove stickiness. A GPS device is the easiest way to get accurate directions to your destination, however you may also want to bring along some maps in case you have an issue with the GPS. You can also trace your route on the map as you go and take notes along the way. You may also wish to consider joining an auto club, especially if your vehicle is several years old, high mileage, or if you are not mechanically inclined.