I would like to work on pset7 using git for version control. I understand the basics of how to use git, but I am wondering specifically how to push to server (on cloud9) the code written on local machine (my laptop)?
If you want to run your own git server on cloud9, I have to warn you that it's a very long process and very easy to screw up. If you still want want to set up your own server, take a look at this from the official site of Git: Git on the Server.
Instead, I would suggest you use GitHub (most popular but your code is public for the free account) or Bitbucket (less popular, same features, still free, your code can be private), for your online syncing, and push/pull your changes to/from there.
If you don't already know it, you can use Git, without GitHub or Bitbucket. You can use it even when offline. Make any changes to want to your code, and when you have internet connection, if you want to use an online repository, push your changes to your online repo. Then if you change machine, pull the latest commit from the online repo, and continue your work.
I use Git all the time and I can't imagine coding without it any more.
If you want a good tutorial for Git, I found this really helpful: Learn Git: Atlasian.