The first time you connect to the repository, the SVN client will require you to authenticate, assuming that your SVN username is the same as your Unix username. If the two aren't the same, then simply hit enter at the password prompt, and SVN will ask for your username as well.
As discussed above, the SVN client will cache your authentication information, so you shouldn't need to enter your password again on that machine. If you check out your project on a different machine, you will need to re-authenticate.