After you have created your account you will need to go to Gmail. From there you will need to go to Settings which is located toward the top right corner. Once you are in the settings you will need to navigate to the Labs tab.
After you have selected the Labs tab you will need to scroll down until you see Text Messaging (SMS) in Chat you will then want to select Enable. Now you will just need to scroll down to the bottom or top and click on the Save Changes button.
Now to start texting your friends all you need to do is go over to the little chat box and type in your friends phone number and save them as a contact. Now you can send text messages to their phone from the comfort of your computer and they can send messages back through their phone. Reminder: This only works for US phone numbers for now but in the future it may work for all phones.