Use secure email services when you can. Use a dummy email address when emailing people you do not trust and make sure that that dummy email is not attached to any of your real information (use a fake name, age, etc). You can also set up emails to reroute, so that you can receive them at your normal address – just make sure that you reply from the dummy account.
This is not particularly secure against government agencies but it is fairly secure against hackers. There are also free, disposable, temporary email services, such as Guerilla Mail, that allow users to create randomly generated accounts that get automatically deleted once the user ends their browser session, or after a time limit.