Do I need to download anything to keep my Macbook Pro "clean"? There are things like "Clean My Mac", "CleanGenius", etc. Are these useful? Are there any that are recommended to use that are free?

I feel like my computer is getting slower and freezing up from time to time. It's not too bad, but I don't want it to get any worse and if I can fix it, I would like to. I have also been trying to delete or uninstall some apps, but they're still in my computer. How can I uninstall/delete things completely and permanently? I think this may be a reason why it is slow. Any help/suggestions would be greatly appreciated!! Thank you!