Straight Talk calls it "Bring Your Own Phone" and has a special website for people bringing their own phone. It is :
http://straighttalkbyop.com/When you follow the link, the first question you're asked is whether your phone is GSM or CDMA. The simplest answer is that Verizon is a CDMA network, but if you're curious about the difference, there's a pretty good wikipedia article about it (http://en.wikipedia.org/wiki/Comparison_of_mobile_phone_standards).Next, you will be asked to purchase a network access code which you'll use to register your phone. The site doesn't provide a link, but the simplest way is scroll to the top of the page and hold your mouse over "Shop" in the banner, then click on "Shop CDMA Access Code". Once you have your code, you can click the red box, which will take you to the next step. It's going to ask you 5 questions in the next section which will require you to have the phone handy. In fact, there are 2 locations you can get that info from. First would be to remove the battery from the phone (when possible) and get that info from there, or you can look up the phone settings (all phones are different, so you'll need to consult your phone's manual for where that is). It is also HIGHLY recommended that you double check to make sure your phone will be compatible with Straight Talk service.