If not yet committed anywhere (git status shows a bunch of stuff modified, it's OK if it's "git add"-ed too):
$ git checkout -b newbranch
Despite the name checkout this usage (with -b) does not check anything out. The -b flag says "create a new branch", so git creates the branch-name and makes it correspond to the current HEAD commit. Then it makes HEAD point to the new branch, and stops there.
Your next commit is therefore on newbranch, which has as its parent commit, the commit you were on when you started modifying files. So assuming you were on master, and you had these commits:
A - B - C <-- HEAD=master
the checkout -b makes this read:
A - B - C <-- master, HEAD=newbranch
and a later commit adds a new commit D:
A - B - C <-- master
\
D <-- newbranch
