Disk Utility will encrypt/decrypt folders. Take your text file and put in into a folder and launch Disk Utility. Select, Disk from Folder under File > New > Disk from Folder. You will be offered a couple of options. It will turn your Folder/file into an encrypted Disk Image.
Terry
If your needs are really simple, the Keychain Access app (Applications-->Utilities) has a secure notes function. Notetaking apps like Yojimbo and VoodooPad have encryption mechanisms as well.
Since you will be uploading stuff online... perhaps an encrypted disk image via Disk Utility would be the best container.