MuseScore is free (open source) notation software that you can download here. 
It’s a great program and quite intuitive once you get past a few initial hurdles. 
We have had oodles of requests for help getting started, so we are doing several segments, each with Q&A to get you up and running.
I am delighted that Helen McManus, one of our students, has created cool templates and key commands for us and will be taking part as well!