1. Empty Your Trash
Even when you put something in the trash, it will still take up space on your computer until you empty the trash. This may get rid of a gigabyte or two.
2. Delete Unnecessary Files
If you have any applications that you haven't touched in months, it's probably high time to get rid of them. Applications take up quite a lot of space, and that space could be used for other things. Videos are also a large culprit in taking up space, as one video can be gigabytes large. If you're using a Mac, go through your Photo Booth, iMovie, iTunes, or iPhoto. If you're using a PC, go through your Windows Movie Maker and iTunes. They can be the largest culprits of taking up storage, as they handle multimedia.
3. Transfer Files to External Drives
If you have some holiday photos that you don't want to delete or want to keep some old footage, instead of keeping them on your computer, it's best that you put them on a USB stick or a hard drive. This way, you'll still be able to access the files, but your computer will have more space. Just make sure not to lose the external drive.
4. Scan Your System
If you've followed all these steps, your computer should have a bit more space. However, you might not know what is taking up all the space on your system. For this, you can use Disk Inventory X for Mac OSX. If you're using Windows, the equivalent to this is WinDirStat. When you open up these applications, it will look through your system and then show you something that looks like this.
Disk Inventory X.
The larger a block is, the more space it's taking up. With these applications, you can easily highlight something that's taking up a lot of space, and delete it. This is probably your best bet for identifying what's taking up so much space on your computer, as it shows you a visual representation of your hard drive. The best part is, both of these applications are free of charge, so you can clear all your junk away without paying anything.