Whether you are using an email client or a web-based email solution, you can encrypt your email.
With the exception of one, these tools serve a singular purpose - to encrypt email messages. Each of them offers a fairly straight-forward learning curve that anyone should be able to get up to speed with quickly. That said, let's dive in and see what each of these tools can do for your encryption needs.
Enigmail is a Thunderbird extension that works in conjunction with GnuPG to encrypt email. This extension requires both Thunderbird and GnuPG to be installed on the machine in order for it to function. This extension works with Thunderbird versions 17-27 on both Windows and Linux. Enigmail also features support for in-line PGP, per-identity encryption rules, automatic encrypt/sign, integrated OpenPGP PhotoID viewer, OpenPGP key retrieval via proxy servers, and much more. Enigmail is available for Windows, Linux, Mac, BSD, and OS/2.
Mailvelope is an extension for Chrome and Firefox that seamlessly integrates with Gmail, Yahoo Mail, Outlook.com, and GMX. With Mailvelope you can generate the necessary keys (which are stored on your local machine) and import other users keys. This extension works from within your web-mail client when you go to compose an email. In the email composition window a small button will appear (in Gmail it's in the upper right corner of the text area of the compose window) that you can click to encrypt the email. By default all outgoing email are all unencrypted, so you have to manually select to encrypt. For anyone who depends upon web mail, this is one of the best solutions for mail encryption.
Infoencrypt is probably one of the easiest means to encrypt a one-off email. All you need to do is visit the site, type the email to be encrypted, type a password (and verify the password), and click Encrypt. The site will encrypt the email and post the encrypted text so you can then copy and past it into an email to be delivered to a recipient. Once the recipient gets the email, they go back to the site, paste the text into the window, enter the password you used to encrypt the email, and click Decrypt. Your email will be quickly decrypted for the recipient to read. It's that simple. Although not for the most ardent of security fanatics, Infoencrypt will work just fine for those needing simplistic email encryption.
4. Mymain Crypt for Gmail
Mymail Crypt for Gmail is a Gmail-specific extension for Google Chrome that makes encrypting your Gmail as easy as a few mouse clicks. Once you've installed the extension all you have to do is go to the Mymail Crypt options (from with the Chrome Extensions window), generate your key, import your friend's keys, and then open up Gmail. From within the Gmail compose window you will find three new buttons (bottom right corner): Encrypt and Sign, Encrypt, Sign. With those buttons you've got all the encrypting power you need for Gmail. Mymail Crypt is my favorite encryption tool for Gmail.
Gpg4Win doesn't actually handle the encryption of email, but if you're going to use an encryption plugin for Thunderbird (or any other email client besides Outlook), this application is a must-install. Gpg4Win not only can generate keys for you, but also help you encrypt files. The installation of this software will install both the GPG system as well as Kleopatra, an easy to use GPG certificate manager. From Kleopatra, you will generate your own keys as well as import keys from others. With Gpg4Win installed, you will find tools like Enigmail not only possible on Windows, but far easier to use.
If you're looking to get quick and easy email encryption up and running, you cannot go wrong with any of the above tools. Not only are they free, they are far easier than trying to get encryption working with Outlook and they won't bog you down with having to purchase and install certificates. Give one of these a try and see if it doesn't meet your email encryption needs.