Oh my sweet summer child, 12 GB is not a lot! :)
well, for one you can start saving webpages you found helpful, maybe your useful links collection or bookmarks, if you have any of those. I would recommend using Firefox and the singlefile addon, or the webscrapbook addon. feel free to look into their settings, but don't let it overwhelm you, of you need it take it in smaller pieces, there's no shame in it.
since this is mostly text, it shouldn't take up that much space quickly, and it's also very efficiently compressible! for example with 7zip.
if you often watch videos, like on YouTube or somewhere else, and you find something useful or otherwise you think it's worth preserving (entertainment is also a valid reason), you can grab it too. have a look at yt-dlp, it's very versatile, very configurable, and not only for youtube.
but this will easily take up a lot of space, videos are huge and not really compressible losslessly.
other than that, have a look at the DataHoarder community: !datahoarder@lemmy.ml (I hope the link works). for even more, you may check the datahoarder and opendirectories subreddits through libreddit/redlib