Check your downloads folder. I currently have 50 GB of data in my downloads folder. Also remember that games take up quite a bit of space. Some games can take 10, 20, or even 40 GB of space. If you do YouTube videos, I'd check to see if you have any clips that you don't need anymore. You may no longer need raw footage for an example. Videos can take up quite a bit of space. I'd also go into add/remove programs, and remove any software you no longer need. If you're not sure what some software is on your computer, then go to http://www.shouldiremoveit.com/ and search for the program. It'll tell you the chances that it's "crapware", and from there you can also see what the program does.

Lastly, I'd talk to your dad about his stuff. It's worth a shot if he has a ton of clutter.

Go to %temp% on your computer, and delete all.. It's safe don't worry, If it comes up with a box saying that the process is running just skip it.

Edit: Mine just gave me 1.26 GB/GiB (not sure) extra.

Disk cleaner. More then likely there's alot of stuff you can still delete. For example in just Minecraft alone: Any worlds you don't play on anymore? Any texturepacks you don't need anymore?

Sometimes when I want to save space and I don't use a large folder often I just zip it.