Pepper spray should last you 2-3 years. If you are unsure whether yours is still good, you can give it a quick spray to see how it sprays (or streams). The oleoresin capsaicin (the active component of the spray) will remain potent, but the propellant will lose power eventually. It can be thrown into the trash, but any remaining pepper spray must be released. Pepper spray is combustible, reactive and corrosive.
Just spray it away from people and animals. You may want to put it into a ZipLock bag before discarding. You may want to contact your local waste disposal facility to make sure it is okay.
If, somehow, you accidentally get pepper spray on your clothes or person, remain calm. Rinse any affected areas with cold water. If you have contact lenses, they may have to be discarded. A saline solution may be helpful if you get any in your eyes; it will alleviate the pain. Remove and launder your clothes, and wash your body with soap and water.
These are just some simple steps to take, but they are important.