From the Textfree App:

Tap on the Option menu at the top of your screen.

 Tap on the Account Info button.

If you don't have a phone number, the Phone Number tab will show "Get One!" When you tap on the Phone Number button, you will start the process to choose a Textfree Phone Number.

Put in your zip code. 

Use your finger to scroll up and down through 10 available numbers.

Once you have picked your number, confirm your choice and your done! Don't take too long to choose a number because someone might choose it just before you.

From Textfree Web:

Head to Textfree Web and input your username and password to log in. If you don't have a number, you should see this screen:

If you click "get new" you'll end up on the screen to pick out your new number. If you click "continue" by accident, don't worry! You can still select a number from the regular page by clicking "options" in the upper right. After "options" is clicked, select "Get One!" next to "Phone number".

Just input your info, and select your new number!

Happy Texting!