Before you start, you must have Mathematica installed on your computer.
Mathematica can be tried 15 days for free from here: https://www.wolfram.com/mathematica/trial/
At the end of the trial, you can purchase a student version from here: http://www.wolfram.com/mathematica/pricing/students.php
I recommend you get desktop only $80 annual license. Check if your school offers Mathematica license for its students. If you can use school's license, then get 6 months license for $55.
You should also install free software Jing in your computer to do your homework. How to install, and use Jing is explained here.