You need to purchase the license directly from the Wolfram site: http://www.wolfram.com/mathematica/pricing/students.php
Did you buy a new computer? This is how you transfer Mathematica license.
We recommend that you choose the time limited one that is renewable yearly or semi annually. You can also get a permanent license. However, considering how often we change/upgrade our computers, and how often Mathematica is upgraded, we feel buying a yearly license is a better choice.