Tag a branch

I am new to GIT. Recently I am encountering a problem related to tag.

I am hoping to apply a tag to a newly created branch. I saw a tag must be associated with a commit. But I don’t find the commit associated with my branch command. Can you help me find this commit? If this is not possible, is there a way to do a commit that affects all the files in the branch so that when I associate the commit with a tag, I can retrieve all the files by that tag.

Note, the project is evolving even in that branch. I am hope to take the snapshot at the branch now before any change happens.