Well, you can do one of two things. Use any old cloud and encrypt it beforehand, or choose spideroak which will streamline it all for you. It costs a bit though.
If you choose the first option, the way I’d do it is to make a private folder which unlocks at login (if the password is the same as your user’s). It’ll store the password in your keyring so it’s not redily accessible. It’ll make a .Private hidden folder which is encrypted, and open up a regular Private folder at login. Whatever you put in the Private folder is encrypted in the .Private folder, which you can upload to the cloud. Will find a link to a guide later, am on mobile atm.
Edit: Here it is, though this is a “Secret” folder, makes no difference. You can even make the encrypted folder any name you want, with or without a “.” for it to be a hidden folder. https://www.maketecheasier.com/create-a-private-encrypted-folder-on-ubuntu-hardy-with-ecryptfs/
Edit II: Or, SpiderOak https://spideroak.com/
Edit III: And of course there are the advice of the more experienced from @Ruffalo