Start up disc is full

How do I know which files to remove from my start up disc

Get and run Onyx. let it do its thing. It will clean out caches and just about everything that is "temporary" that you don't need to keep. Make sure you get the right version for your particular version of OSX. Onyx

Once that is done, if you still need space, the first thing most people do is move pictures/movies, etc, to external storage devices and keep them off the internal drive.