From here on out, you'll be working on full (if small and limited) applications, which have many more supporting files (like controller and model files, as well as storyboard and asset files) than the first lab required. In addition, starting with Lab 3 and going to the end of the class, you'll be working on one larger application, adding features every week. So, you will submit your work from here on out by providing me with a repository link to a (private - see here for information on how to get these on github with a student email address) repository on github or bitbucket. Hopefully you're at least a little familiar with using Git for version control, but to submit assignments here you'll need to learn a little bit about how Xcode integrates with Git.
I'm just going to go over the extreme basics (creating a repository for an app, making commits, and adding/pushing to a remote repository), if you would like more information on how Xcode integrates advanced Git features like branches, reverting to old versions, etc, this is a pretty good article.
When you create a new Application project (As you'll do at least twice in this class), Xcode will prompt you to pick a location to save the project. This will create a directory for your project at that location. At the bottom of this window is a checkbox labelled 'Create Git repository on my Mac' - if you check this, Xcode will initialize the project's folder as a Git repository. Do this when you're creating your new projects for this class. Xcode will also perform an initial commit of the default files your project comes with - you can view this commit by going to the Source Control Navigator (Command-2) and looking at the master branch.
When you change and save a file, you'll notice that it gets a little 'M' next to its file name in the navigator - this means it's been modified since your last commit. To commit the changes you've made (after you confirm they haven't broken everything), just go to the Source Control menu at the top of the screen and press 'Commit' (or hit Alt-Command-C). You'll be taken to a screen which summarizes the changes that have been made since your last commit and allows you to choose what should go in this new commit. By default, all your changes will be selected. You can choose not to commit some files, or even discard changes in the diff that it shows you. When you're done, you can hit enter a commit message at the bottom and hit 'Commit 1 File' (or however many) to make your commit. If you have remotes available, you can also push them right now with the little bottom-left checkbox.
For the TAs to grade your work, you'll need a remote repository to push your changes to. GitHub integration is really easy with Xcode - You can add a GitHub account in the Xcode -> Preferences... menu (Accounts tab) - hit the + and select 'GitHub', then enter your credentials. You can then switch to SSH authentication, if you have that set up for GitHub.
Creating a remote is easy if you've done this - right click on the project name in the Source Control navigator (left pane, second tab) and select 'Create "" Remote on GitHub...' - then you can choose a repository name, whether to make it public or private, and a Remote Name, and Xcode will add the remote for you, as well as pushing any commits you've already made.
This is all you need to know for submitting your work, but if any of it was unclear or you feel like you want to learn/use more advanced features of Git with Xcode, I highly recommend this tutorial.