Oh lawds, please let someone set this straight before I see anything else said here.
Everyone with some form of an intelligence is capable and has the ability to use magick, depending on how you look at it everyone uses it in everyday life, some of us however are more aware of where and when it is used. As to casting, not every single spell will work straight up - casting is something which takes practice, knowledge, and in most cases a little research and understanding.
As to where to begin the most common point you'll come across is to look at the basics. I'd advise however you look at 3/4 lists of these before making your final decisions. Last but not least, actually the most important, is to research, read, and study. Use those three points as well as some good old common sense and you're on your way.