How do I set up a Virgin Mobile prepaid plan on a used phone that was given to me?

Ok, my fiancee just got a used Virgin Mobile Blackberry phone with no plan and no packaging, literally just the phone and nothing else. How do we go about setting up a prepaid plan on it. We want the $25 a month, 300 minutes, unlimited texting plan. The serial numbers and whatnot are underneath the battery, so we have all that information. Forgive us, we're just a little "cell phone illiterate" lol. (No trolling please!)