My Lenovo says the hard drive is full, yet I'm only using 780MB of space?

I'm not a computer genius, however I can do the basics. I bought this computer for hella cheap, $150 two years ago, and had problems from day one. The One Drive has 30GB of space, and yet 27 of them are being used… By what? I'm at a loss. I deleted Kasperskey, which made is go MUCH faster, and since only use the computer to write… That s it. I have deleted EVERYTHING I CAN delete, and like I said I only use 780MB of space. I can't even download safari or any other search engine. I'm at a loss of what to do. I also thought my computer only had 2or 4GB, but when I check out the hard drive it has room for 30… What piece of this puzzle am I missing, and what can I do, other than to buy a new computer?