You should not need to use sudo to run
svn mkdir. The repository has its own permissions, but subversion should handle those for you; the subversion command line interface (or any other interface such as Tortoise SVN) should be public and should not require superuser access to use. When you check files out, they should be owned by you.
svn mkdir runs in two different modes: it can act directly on a URL, in which case the it also commits the new directory, or it can act on a local working copy. Here's what
svn help mkdir has to say on the subject:
mkdir: Create a new directory under version control.
usage: 1. mkdir PATH...
2. mkdir URL...
Create version controlled directories.
1. Each directory specified by a working copy PATH is created locally
and scheduled for addition upon the next commit.
2. Each directory specified by a URL is created in the repository via
an immediate commit.
In both cases, all the intermediate directories must already exist,
unless the --parents option is given.
If you're not seeing the results that you want when you try running
svn mkdir URL, try checking the repository out, running
svn mkdir to create the tags/branches/trunk, then committing the changes. At the very least, this should give you more information if anything goes wrong.