Of course, to start to use the app you'll first need to create a ProtonMail email account. Creating an account is totally free, and it'll only take you a couple of minutes to make one. What's important, though is taking enough time to choose a good password and making sure your back up email account is up to date.
When opening an account with ProtonMail you'll get 500 MB of free storage space, that you'll be able to increase by making a donation to its developers. As is to be expected, the app offers all the services that are usually included with a high quality email client, on top of a bunch of other specific security features. For example, you'll be able to use ProtonMail to send emails that are password protected, or emails that self-destruct after a set amount of time.
Android 4.03. or higher required.