TV and monitors use completely different scan frequencies - in order to do it you need to first decode the composite signal, digitise it into memory, then read it out at a differnt rate - oh, and also scale it up to match the screen resolution. It's far from a trivial project, and you can buy units that do it - but the picture quality is usually pretty poor anyway.
A far better idea is to sell the monitor, and buy a TV - the picture will be FAR better, and will probably be cheaper as well.