You have to first unlock your device. If its an AT&T device (HTC Inspire 4G), then it is locked to AT&T's network. To unlock your device, you can research unlocking it, call AT&T if you are a long time customer, and they will unlock it for you. But even before you unlock it, if you are leaving your AT&T network under a 2 year (or grandfathered 1 year) contract, you must either finish your tenure with AT&T until contract end date, or pay an Early Termination Fee (ETF) which starts at around $300-$375 and shrinks as you complete each month. Check with your carrier, AT&T about your Early Termination Fee.Once this is completed, you must some way figure out how to unlock it- you can call AT&T if possible. (If you have an iPhone, it's easier, believe me!) Once you somehow figure out unlocking it, then purchase the SIM card of the carrier you want to choose, in this case, Straight Talk. Then, insert it into the SIM slot of your device. Your device, if unlocked properly, will function on StraightTalk's Pay-as-you-go EDGE ('2G') or maybe 3G network, if you're lucky. It's also possible that the data network could be un-functional, but I highly doubt it. By the way, don't be assured that MMS will work, because it probably won't.Note: I am writing this in the point-of-view of unlocking an iPhone (Apple fan:), and partially assuming that the process will be the same, and I'm sure it is.By the way, this process in no way illegal, but it is wrong. I would prefer buying a StraightTalk phone instead. Or please consider an iPhone.