There are quite a number of file/folder encryption programs on the Mac App Store. I seem to remember getting one called BitClamp in a bundle a long time ago. Also remember one called Cryptor. Hider 2 from MacPaw is another option.
Sterioscopic was a bit short on instructions. Go here for full instructions on how to do this. How to create a password-protected (encrypted) disk image - Apple Support
If you choose not to save the password in key chain (which would sort of defeat the whole point) make sure you can NEVER forget it. If you do there is NO way to get the contained info or indeed the storage space back. Not even trashing it.
Call me paranoid but I'm a little bit wary of using other developers apps to encrypt data. I fear that the app might crash or become incompatible for some reason not to mention paying for it (I admit to being a bit of a tightwad).
Creating an encrypted .dmg (Disk Image) has worked very well for me. When opened it mounts on the desktop like a Disk Drive, you can drag files into and out of it and open it just like a standard Finder window. No frills but native and very reliable.
I explored the issue and by far the easiest way it to simply turn on the encryption feature for the entire computer. Too bad Apple isn't like Windows where all you have to do is right click a file and crate a password. No, that would be too easy!
That is something Windows can't do either without doing what the Mac does, which is zip it and then apply a password to the zip file.
btw IMO you are asking for trouble by encrypting your entire computer.
When Apple applies encryption, that is a very strong encryption, and when you inevitably can't get back into it you'll be here begging us to give you a way to crack what you wanted nobody to be able to get at.