You might want to read the Oauth spec:
http://oauth.net/core/1.0/
The oauth signature is generated from the very params you use in the base string (request URI) using the algorithm you listed there (in this case HMAC-SHA1).
We also have a testing tool here:
http://wiki.developer.myspace.com/index.php?title=OAuth_Testing_Tool
..as well as another discussion on Oauth flow here:
http://wiki.developer.myspace.com/index.php?title=OAuth_REST_API_Usage_-_Authentication_Process
you use your consumer secret to generate the oauth_signature and you append it to your base string so our servers can verify this request is legitimate.
thanks,
Joel