Encrypted how? There are a lot of different mechanisms for that. What are you trying to do? Send messages to other people? How secure do you need it?
Probably the simplest method is a rotation cypher - rot13 is common. Substitution cyphers are common too - although you need a way to send the substitution key to the other person. One time pads are a common way to do that. Benefits here are that you can do the encryption and decryption by hand.
From there you can go to more intense methods of encryption but they'll typically require computers. That depends on security then. There are certainly websites where you can enter some text, your keys, and get the text back. Then your friend just needs the decryption key and they can use the same website. That said - do you trust the website not to capture the data? If it's just for fun it doesn't matter but if it's more than that you need to think about how you handle that situation.