Apple - TrueCrypt alternative for Mac OS X

Just use FileVault 2, which comes with El Capitan and is pretty secure.

If you need separate volumes or passwords for different projects, you can also use OS X encrypted disk images with 128 or 256 bit AES encryption. This allows you to choose to store the passphrase for each volume offline, in separate keychains or in the main keychain as you see fit.

The benefit of FileVault is whole disk hardware encryption so you should start there and then layer in encrypted disk images for projects that require even more protection or isolation.

VeraCrypt

If you want operating system independence, e.g., a thumb drive that you can use on systems other than macOS, you might want to look at VeraCrypt - the successor to TrueCrypt. VeraCrypt is open source as well. Further information can be found in Encrypt FAT USB drive under El Capitan.

I personally use encFS, which is pretty easy to install on MacOSX, given that you know how to use homebrew and casks:

brew cask install osxfuse
brew install homebrew/fuse/encfs

To use it, I theen use two aliases, one to mount the volume:

mypasswd='encfs ~/cloud/Documents.encfs ~/Private -- && cd  ~/Private'

and then to unmount (and close the terminal):

mypasswd_umount='cd ~ && umount ~/Private && exit'