If you want to import your existing SVN repository at Savannah (preserving all history):
Prepare a dump (see below):
svnadmin dump myproject-svn/ | bzip2 > myproject-dump.bz2
Upload it at Savannah (using your download area at download.savannah.gnu.org or download.savannah.nongnu.org):
scp myproject-dump.bz2 email@example.com:/srv/download/myproject/
Or anywhere else that is convenient.
Enable SVN for your project (in Main -> Select Features from your project page)
Post a support request and specify where your dump was uploaded:
The Savannah Hackers will then import it on the system (Savannah hackers: see AdminSvnImport)
Do not send a tar.gz of your SVN repository. Using a dump is important because it's independant of the subversion release.
Note: if you just want to upload your initial source code, not your whole project history, just use 'svn add' and commit.