Check git status: Menu -> GIT -> Status (M-xmagit-status). If you sure, you can add files from this screen (just set cursor to file name and press s, like "stage")
If you not sure, that all changes should be added to git, than open diff to add selected changes: Menu -> GIT -> Manage Changed (M-xmagit-diff-unstaged). Than set cursor to diff and press s to add it, or k (like "kill") to discard it.
Commit added changes: Menu -> GIT -> Commit Changed (M-xmagit-commit)
View git log (and rebase): Menu GIT -> Log Current Branch