Welcome to the Forum, Citatus.
There are others who can give you more technical information, but the .sims3pack files probably aren't the best things to delete. For example, the items in our Swap Shop are .sims3pack files, so I would assume that if I were to delete them, I'd lose whatever I've downloaded. There are other areas you can
clean up first. An addition, you may want to consider deleting pictures and music on your computer. That can take up a lot of space.
I'm sure you'll get many more suggestions before much longer.