This process can be a very time consuming especially if this is your first time you are doing this type of migration. Before you get started, you have to have good backups of your database as well as your repository. Worse case scenario, if something does go wrong during this process, you can drop everything and start over..
Here is a good tutorial about Migrating Microsoft SQL Server Database to Oracle Database 11g:
Good Luck to you !
When you say you tried to delete or truncate the repository then was this successful ? Could you then create a new repository ? Make sure the repository owner has all the necessary privileges as detailed in the documentation.
If you have successfully created a repository then what problems do you have that stop the migration working ?
If the SQL*Server database is very large (more than about 50mb) then you should make an offline migration which gives you more control over the migration.
Have a look here at the SQL*Developer migration site -