The first thing you need is a video capture program.
Fraps and bandicam are good for starting. Fraps has a free trial that only records 30 seconds of video. Paid version records endlessly. Bandicams free version records 10 minute video and displays a water mark over captured game play.
If you have a Nvidia graphics card you should be able to use Geforce Experience Shadowplay. It records endlessly.
You will then need a editing program such as windows movie maker (for beginners). Sony Vegas is a good program to use but expensive. Lightworks is another free program but has a 30 day trial.