In general, you want to avoid writing down sensitive information and just keeping it in your head, but that's not always an option. If you have to write down a password, or some other sensitive information, you can use a Caesar Cipher to quickly encode and decode that message on your own.
Caesar Ciphers are very simple methods of encryption, because they work by shifting the alphabet over a few characters and matching up the letters (see the picture above) — in fact, if you've ever used any cypher as a kid, it was probably a Caesar Cipher. This cipher turns a statement like "The quick brown fox jumps over the lazy dog" into this:
WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ
The first letter is a W because it's three letters after T. K is three letters after H. H is three letters after E. Basically, you can easily decrypt that message by just rewinding the alphabet by three letters. It's easy to decrypt if you take some time to figure it out, but it's plenty to keep non-motivated people from spying on your stuff.
This is just one useful personal encryption method of many. For more, read this guide.