How to add tags to my sandbox module?
In git, the terms "branch" and "tag" always applies to the whole repository, not to the individual files.  A branch denote is a series of commits (so you always use an "x" instead of specific version number for a branch). A tag is a label for a specific version, so you specify the version number (e.g. 1.0 for the first released version, 1.1 for the next, and so on.)

The command git branch -v will give a verbose listing of the current head.  It should, in the case of Drupal 7, start with a line * 7.x-1.x.  If it says master on the first line, you've not set things up right.

Before adding tags, you need to get your project on a 7.x-1.x branch.  In your local repository, do the following to checkout branch 7.x-1.x and delete branch master, and push those changes to the remote:

git checkout -b 7.x-1.x
git branch -d master
git branch -v
git push origin 7.x-1.x
git push origin :master


After you've made sure the branch is 7.x-1.x, you can go ahead and add a tag to your release.

You can use the commands you refer to, to tag.  To see all the tag(s) in the local repository:

git tag


After pushing to the remote, if you've done this correctly, you should be able to click on the Version control tab and find the 7.x-1-x branch in the Version to work from: pull-down menu.  If you retain the master branch, make sure that you make the 7.x-1.x branch the default.