The commit or ci ("check in") command tells CVS to make the repository look like your local copy.
If multiple people are using the same repository, it's a little different, but for now, we assume that you're the only person using the repository:
cvs commit foo.c
or:
cvs ci foo.c
When you do this, CVS starts an editor to let you enter a description of the file. Type in something meaningful, such as "A file to test the basic functionality of CVS." This is completely free-form, so you can add whatever message you like. When you're finished, save and exit. CVS then tells you the file is committed.