Electrical issues, battery draining

I bought the car listed above about 2 years ago, and it has had many electrical problems since. Headlights randomly dimming, internal lights sometimes not working, slow power windows, etc. Recently my car has been not turning when I turn the key in the ignition, with only a small click when I turn it initially. I measured the voltage of the battery and when it is off I measure about 11.8V, and when it occasionally turns on I get 14.2V. I'm thinking either a bad alternator or some form of electrical draining is going on, I would appreciate some help with this one. Thanks.