Let's say that your vehicle isn't pre-rigged for towing. Don't worry, you can still take your boat to the lake for the weekend. You just need to buy a trailer wiring kit and do it yourself or have a professional do it for you.
The easiest way to find the wiring kit you need is to order it on the Internet or go to an auto parts store that has a nice selection of wiring systems. The kit will be specific to your vehicle make, model and year. These days' kits take most of the brain work out of it, and you won't need to wire anything yourself. That means no splicing, soldering or even taping of wires. What you'll get in the kit will most likely be a T-harness or T-connector. The T-harness has two plugs, one for each taillight. These plugs are connected to each other and feed into a single plug that hooks to your trailer. Each side of the T-harness will plug into your taillight, and the original plug will then go into the newly installed T-harness.
Once you have the right T-connector for your vehicle, it's simply a matter of plugging it all in. What you want to do here is access the rear taillights of your vehicle from the inside. If you have a wagon, you can do so by simply looking under your trunk. Lift up the carpet in the rear and you'll find a storage area where your spare tire and jack are most likely kept. There may also be a piece of molded plastic that shields the taillight from the rest of the trunk. You need to remove this as well -- it most likely pops out and snaps back into place pretty easily. Some SUVs won't have access to the light from the trunk, so you'll have to remove the entire taillight housing with a screwdriver.
Once you have access to the taillights, you'll notice the wiring plug leading into it. This comes directly from your car's electrical system. All you need to do is unplug it from the light, plug your T-harness in and then plug the car's original set of wires into the newly installed trailer harness. Do the same thing on the other side, and you'll be wired for towing your trailer. Then simply attach the ground wire to some metal. It will probably be a ground with a metal loop at the end. Just drill a hole into the metal base of your trunk and drive a screw through the loop to attach it to your car.
The plug that you'll attach to your trailer will have several feet of wire. You can just keep the plug and wire stowed under your trunk storage space alongside your spare and jack until you need it for towing. When that time comes, simply pull it out and close the trunk on top of the wire -- you won't do any damage to the system.
If you find that you have a car that's too old or a trailer that has a mess of wires, you may need to bring your vehicle in for some help from a mechanic. Otherwise, it's as easy as plug and tow.