To get started:

sudo gem install bundler
bundle install
bundle exec jekyll serve --drafts --config _config.yml,_local.yml

The _local.yml bit excludes some large directories which you're unlikely to change.

To write a new post, put it in _posts/YYYY-MM-DD-post-title.md. The front matter should look like:

---
layout: post
title: Title of the blog post
time: 12:36PM EST  # displayed literally on the home page
datetime: 2016-01-19 12:36PM EST  # parsed and placed in atom.xml
summary: "Summary that goes on the home page"
---