There's some tools offering transparent encryption, i.e. encryption is running in the background and you don't notice it in your daily workflow.
There's VeraCrypt, which is great for using it locally. In a cloud environment, it has some usage problems, see here. Namely, it can cause re-uploading all your files even if you only changed some few, and it can cause nasty synchronization conflicts when you don't wait for it finishing synchronization before you edit your files on other computers.
A tool I used to encrypt my Dropbox for quite some time is EncFS. Although it encrypts files individually (and does not hide directory structure or file sizes), it's a great tool and does the job. Unfortunately, there has been a security audit showing that EncFS is insecure when used together with cloud storage, see here for a detailed explanation.
There is a relatively new tool that was designed specifically to be used for encrypting cloud storage. It's called CryFS. Disclaimer: I'm one of the developers. It encrypts file contents as well as directory structure, metadata and file sizes (see here for why this is a good idea). It follows established security standards and there is a scientific thesis proving its security, see here.