Here's something I found super helpful if you've already been messing around with your repo before you asked for help. First type:
git status
After this, you should see something along the lines of
On branch master
Your branch is ahead of 'origin/master' by 2 commits.
(use "git push" to publish your local commits)
nothing to commit, working tree clean
The important part is the "2 commits"! From here, go ahead and type in:
git reset HEAD~<HOWEVER MANY COMMITS YOU WERE BEHIND>
So, for the example above, one would type:
git reset HEAD~2
After you typed that, your "git status" should say:
On branch master
Your branch is up to date with 'origin/master'.
nothing to commit, working tree clean
From there, you can delete the large file (assuming you haven't already done so), and you should be able to re-commit everything without losing your work.
I know this isn't a super fancy reply, but I hope it helps!