I actually have ATT - I had the unlimited data grandfathered into my plan.because I had the iPhone.You have two options:
Your first option is to add T-Mobile's Wi-Fi tethering plan to your account, which is an extra $15/mo. Your second option, which is what I did (I have a Galaxy S 4G), is to root your phone and install an app that allows Wi-Fi tethering with root access. I had no clue what I was doing when I rooted my phone, but it turned out to be a lot simpler than I expected.
I assumed you are with T-Mobile. That in mind, know that T-Mobile has a data cap of 5 gigabits. Fortunately they don't charge extra for going over, but they will switch you from 4G to Edge (aka 2G) for the remainder of that billing cycle if you do happen to cap out.
Thanks for your review. So is the general consensus that the only way to tether by rooting your android phone? I was hoping there was an easier wayWireless tethering is the only way to access your phone's data connection via Chromebook at the moment. PDAnet works great for Windows machines, and is also a lot easier than rooting your phone, but Chromebooks don't have the driver support needed to use such a program.
I wrote a full review on my experience with my new Chromebook that can be read here. It covers data tethering and more.