First, create your project then upload your unversioned source code using svn client:

svn import sourcedir https://myproject.googlecode.com/svn/trunk --username myaccount

- when prompt for authentication, get your password from the ‘Source’ tab

Get a working copy of your versioned source code:

svn checkout https://myproject.googlecode.com/svn/trunk/

If you’ve made changes on your working copy you can apply those changes on the repository by simply this command:

svn commit -m "Put a descriptive comment."

- you have to be inside your working copy/directory.

And if by chance you forgot or simply wanna change your comment on that revision, you can add or change it:

svn propset --revprop -r 26 svn:log "Fixed someMethod() parameter."

If multiple developers are currently working on the project/repository, you can use this command to update your working copy:

svn update

- this will incorporate their changes into your working copy.
- again you must be on the working copy/directory.