1. I recommend watching the following Microsoft video:
Video: How To - Migrate from VSS to Team Foundation Source Control
2. Make sure you have installed TFS 2008 Service Pack 1 on your TFS server. See http://blogs.msdn.com/tfs_migration/archive/2008/09/08/improved-vss-converter-available-with-tfs-sp1.aspx for information on VSSConverter improvements.
3. Read http://msdn.microsoft.com/en-us/library/ms400803(VS.80).aspx and http://msdn.microsoft.com/en-us/library/ms400685(VS.80).aspx to learn about the Analyze and Migrate commands for VSSConverter.
4. Read http://msdn.microsoft.com/en-us/library/ms181246(VS.80).aspx and do the five step process to prepare for migrations: (a) Back up your Visual SourceSafe database. (b) Identify and resolve data integrity issues in your existing database using the Visual SourceSafe Analyze tool. (c) Run the converter tool to identify potential sources of information loss. (d) Specify which Visual SourceSafe folders to migrate. (e) Create a user mapping file to map Visual SourceSafe users to Team Foundation users.
5. Read http://msdn.microsoft.com/en-us/library/ms181247(VS.80).aspx and do the two part process for migrations: (a) 7 steps to modify the settings file to create a migration file, (b) 5 steps for running the converter. There's actually a 6th step that's essential for developers using VSTS. See http://msdn.microsoft.com/en-us/library/ms253188(VS.80).aspx for instructions on migrating source control bindings.
6. Be sure to search the MSDN forums (see http://tinyurl.com/vss-tfs-help or http://social.msdn.microsoft.com/Forums/en-US/tfsversioncontrol/threads) for the latest discussion threads on VSS->TFS migrations. Also see http://search.twitter.com/search?q=VSS+TFS to check out the latest gossip about doing VSS->TFS migrations.
7. Be sure to google VSSConverter for further information on the migration tool. I did a search today that provided a few interesting links including http://www.codeplex.com/VssConverterGui.
8. If you really want total flexibility then learn about the Microsoft.TeamFoundation.VersionControl.Client namespace that opens up possibilities such as http://blogs.msdn.com/edhintz/archive/2006/02/03/524312.aspx, http://www.codeproject.com/KB/DLL/MySidekicks.aspx, http://tf4mono.googlecode.com/svn/trunk/class/Microsoft.TeamFoundation.VersionControl.Client and http://www.sashasydoruk.com/2007/02/14/tfs-howto-get-a-list-of-users-and-pending-changes.