Below are some links that should enable you to access Sage, the free open-source computer algebra package.

Introduction to Sage: Download the textbook Sage for Undergraduates by Gregory Bard, published by AMS (electronic edition available for free at http://www.gregorybard.com), or see the documentation at the official Sage website http://www.sagemath.org-
Sage as a calculator : to evaluate short commands one by one, the Sage Cell Server https://sagecell.sagemath.org is easy to use. Just type in the command and click "Evaluate". Programming with Sage : For larger projects, programs, etc try the Sage Math Cloud https://cloud.sagemath.com- Lastly, you may download your own copy of Sage from the official Sage website http://www.sagemath.org but this is much more hassle and has fewer capabilities than the Sage Math Cloud.