Every time I export a new composition, it lags when I try to look at it in windows media player.The files that get rendered are usually pretty big (several hundred megabytes or a couple gigabytes). How do I fix this without it looking awful? I tried using the Prism video converter and it ended up with reduced quality and my background color changed slightly for some reason in the preview. Also, shouldn't it be built into the software? Shouldn't there be no problems? I must be doing something wrong.
You are using the default lossless format which is not designed for playback but design to create a digital intermediate for further production. You need to learn the basics. Start here:
Watch the third video on rendering.