What is the best free screen recording program.?

I have a macbook pro and I do not have enough money for I show U. Plus I downloaded a program called desktop recorder and it won't work because every time I try to record it saves as soon as it starts which creates a 0 second video every time. I would like to know what a good free screen capture program would be because I would like to start making youtube videos. However if you can fix my problem with desktop recorder then that would be preferred.