Take your car to a reputable repair shop or brake specialist and have the brakes inspected. If the pads are indeed worn down, the pad replacement is in order along with the turning of the rotors to restore the proper surface finish and remove any ridges on your Nissan rotors that may have formed on the surface during normal wear. This is not an overly expensive process. If your car is rear wheel drive, it's an excellent time to have the front wheel bearing cleaned and repacked with the required grease lubricant. If it's front wheel drive, then pad replacement and rotor turning is all that's required. Depending on your area and the type of brake pads you would like to go with, the job should run you about a $150 job start to finish.
For the Nissan Altima, I would recommend a full ceramic brake pad and Akebono full ceramic pads are the best, In my opinion. It is true that the Akebono ceramic pad is a little more expensive than a traditional brake pad, the Akebono ceramic pad will give you longer life, and produces less heat resulting in better performance and longer life on your rotors as well. If you would like to install your new Akebono pads on your Nissan Altima yourself, we have found an excellent site that will help you install your new pads, saving you alot of money at the repair shop. Go here for a full color instructions on changing your own Akebono Brake pads.