You can just replace the pads, but you won't get the best performance out of your brakes in doing so. You need to inspect (or have inspected) the vehicle to see what is going on. Lots of times when you hear an obnoxious noise from what sounds to be the brakes, it is the "squealer" (small metal strip on the brake pad) which, when the pad gets thin enough, comes in contact with the rotor and starts making a squealing noise (thus the name). When you get to this point, you really need to start thinking about getting the brakes replaced ... if not only for the safety aspect, but for your sanity as well (listening to the squeal will drive you over the edge ... pun intended :o)).
The best route if the brakes need to be replaced is to replace both the rotors and the pads. I personally find this easiest, as well as take less time to get it done (since I'm doing it myself). If you are mechanically inclined enough to do the job, then you can change them out without too much hassle. It saves a trip to a shop to have your rotors turned, as well as allowing you to do the work on your time and not pay the shop costs in the process.
The next best is to have the rotors turned and the pads replaced. If you have a shop do the replacement for you, they'll tell you whether or not there is enough meat left on the rotor and can advise you on replacement or turning, whichever suits your needs.
The next option is to replace just the pads. I HIGHLY recommend you do not do this, as your stopping power is greatly diminished. Could it get you by in a pinch? Sure. It's better than going metal-on-metal in your brake system, but only because you won't be damaging parts while using them. You're not really doing yourself a favor, or your car. More than likely you'll burn through the pads much faster than you would with fresh surfaces on your rotors, but it can be done.
Your final option is to drive it as it is until the wheels fall off. This really isn't a "viable" option, but it is an option. I'd highly recommend against this option as well (for painfully obvious reasons). If you were to make a choice like this, you're not only making this choice for you and your car, but for any passengers and possibly other vehicles/drivers/pedestrians around you. This would be a very negligent option.