GrandPerspective is a nice app that shows a graphic representation of what's on your disk, so you can get an idea of what is taking up too much space.GrandPerspective
Don't buy CleanMyMac or any other dreck that offers to "clean" your Mac for you. Worst software ever.
If there are apps in your Applications folder you don't need any more, drag them to the trash and empty the trash.
In general, it is media files, like music, photos, and especially video that is taking up a lot of space. You can move your iTunes library, iPhoto Library, and iMovie video to an external drive.
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How do I remove stuff from my startup disk?
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