Proper way to build plugin

Very sorry if the info I need is somewhere obvious.

Basically, I am creating a plugin and it is now 1116 lines long in one file. I am calling a css file as well, but those are the only two files total. I am using URL variables and $_GET to create pages within the admin area. Is there a better way to do this? Should I split up the pages into different PHP files and then include them? Other suggestions?

I am not having any problems doing it this way, but it seems kind of wrong.

Thanks for any suggestions!