stage.new

git add filename
or
git add --all
to add new files to staging area, than those files are
call be tracked.

How to Unstage?
git reset HEAD <file> ...
HEAD here refer to last commit in current branch.
note that <file> still reserve their changes.
If you want to discard the changes.(staged file -> unstaged and unchanged)
git checkout -- <file>

git diff
to see unstaged change to last commitment.

if you staged, then use
git diff --staged
instead.

If you found that last commitment you just did are wrong, like forget a file to
commit, you can undo last commitment, back to status that just before that commitment
by runing:
git reset --soft HEAD^
Then you can find everthing back to staged.

git reset --hard HEAD^
like the pervious one but discard the changes also.

ONE LAST WORDS: DO NOT USING CHANGE HISTORY COMMAND LIKE
git reset --soft HEAD^
ONECE YOU PUSH.