Email is usually/historically encrypted using
GPG. There are
GPG addons for most common browsers; For example GPG Extension for Firefox.
You will need to generate a key-pair for yourself and follow basic key-management procedures. This is all fairly long-winded to put into an answer on here, but there are some GUIs available that make the process easier, such as GPG4Win.
The major caveat with all this is that you will need to convince your friends to participate. They have to decide to send you encrypted email, and likewise they have to set up a key pair themselves in order for you to be able to send them encrypted email.
This lack of a ubiquitous
web of trust subsystem is why email encryption is still very much underadopted. The widely used PKI doesn't lend itself so well to email, as it lacks flexibility and is prohibitively expensive.
Email at its core is just raw text transfer. That doesn't change when you start encrypting things. Typically the body of the message is encrypted using the public key of the recipient. This encrypted message is formatted into printable characters (usually with base64 encoding), this stage ensures that the encrypted block, which might contain weird and wonderful bit-sequences that map to weird, unprintable characters, is transferred safely by encoding it in simple characters (base64 uses the alphabet, numbers and a few common symbols).
The browser addons are able to identify this encrypted block inside the web page, and will allow you to decrypt it, which returns it into a plain text format.