Find your Textfree number in the app by visiting the Options menu. This can be done by tapping the "gears" icon (press your device's "Menu" button first if you're using Textfree for Android).
Your phone number will be listed in "Account Info".
If you don't have a Textfree number, or if your number has expired, you'll be given the option to "Get one". Tap on this to get a phone number for your account. This process is the same whether you're on an Android or iOS device.
What if I'm using Textfree Web?
No problem! If you're logged in online, simply click "Options" in the upper right corner of your browser for "Account Info". Your phone number will be displayed there.