Branches in SVN are essentially directories; you don't name the branch so much as choose the name of the directory to branch into.
The common way of 'naming' a branch is to place it under a directory called branches in your repository. In the "To URL:" portion of TortoiseSVN's Branch dialog, you would therefore enter something like:
(svn/http)://path-to-repo/branches/your-branch-name
The main branch of a project is referred to as the trunk, and is usually located in:
(svn/http)://path-to-repo/trunk