Many new cars have a trunk release lever on the floor below the driver's seat. These cars should have a cable that runs from the release lever to the trunk. Look for the cable beneath carpeting or upholstery, or behind a panel of sheet metal. If you locate the cable, pull on it to release the trunk latch. Look for a tool in the trunk.
Many cars have emergency kits inside the trunk, underneath or with the spare tire.
These kits may contain a screwdriver, flashlight, or pry bar. Use a screwdriver or pry bar to pry the latch open. You can also pry the corner of the trunk lid up and wave and yell to signal passersby. Dismantle the car's brake lights by yanking wires and pushing or kicking the lights out. Then wave and yell to signal passersby or other cars.