If you don’t know where the downloaded install file was saved to, it’s probably in \Downloads. Next time you save something, don’t try to save some time by not looking at what you’re doing. The dialog window is there for a reason, to inform you. If you actually managed to install it and started a document and can’t find where it was saved, it’s probably in My Documents. Restart Libre, the opening window lists the most recently opened/created files, and yours should be there.
On a side note, if you are a complete novice to computers, you’re probably better off with Windows than with Linux.