I'm very new to the Internet and accidentally have come across 'Bittorrent'?

I've read a lot of information online about it, and it sounds great, but each time I've tried to download anything I've been asked to register ... fair enough ... but it always involves a fee. I don't mind paying for a decent service but does anyone know of any try before you buy sites? There are so many, I want to be sure that a membership fee will not go to waste. I want a site to suit my specific needs (basically films for the grand-children, if I'm honest!) Any help would be much appreciated. Thanks, Tracy