To test locally, you need to have a recent-ish version of ruby installed (ruby 2.4 or newer). Run the following to get up and running:
gem install bundler
bundle install
bundle exec jekyll serve -l
That should install jekyll and its dependencies, then start a live server.
If you write a simlpe page, often in markdown for the website, you will probably
use the default layout. It has a single row and a 12 column div ready for your
content. If your page has multiple rows, the main layout is what you want. In
this layout, you are responsible for creating your own divs for the rows and
columns.