Q: how can i remove old files from start up disk

Go into your home folder and remove things you don't need. Either copy them to an external drive or optical disk if you want to keep them, or just trash them if you don't. Make sure you empty the trash when you've finished.

Also, uninstall any Apps you don't use. If they were Drag-and-drop or App Store apps, you can just drag them to the trash or Uninstall them in the App Store>Purchases. If they had an installer, get the uninstaller from the company that created the app.