Even if there is only a master branch now, in your terminal, you can make a sub-branch with:
git checkout -b 'subBranchName'
Then push that branch into GitHub:
git add .
git push --set-upstream origin subBranchName
Go to that repository in GitHub and click on branches. There will most likely just 2: the default master and the new one you created. Click on "New Pull Request", then you can compare the changes there. I prefer this method, because you can share the info with your team and have them approve or reject it. You can also take notes and make more edits all within GitHub.
VonC's answer with fetch and rebase will be a lot quicker and easier, but adding a branch and merging it will also teach you a lot about GitHub. As a beginner myself, I highly recommend branching to easily discuss with your team, find mistakes, and backtrack if necessary.
Another tip is hit
Ctrl `
while in VS to pull up the terminal.
Make a goal to push to GitHub daily, even if you push a messed up branch that you plan on deleting without merging. You'll get used to in in about 2~3 weeks.