Most transactions will be signed automatically by the node, if it has the corresponding private key in its wallet.
If you need to sign transactions using external keys, the following tutorial may help you:
http://www.multichain.com/developers/external-key-management/
Or if you need to sign transactions by multiple nodes (because you are using multisignature transactions):
http://www.multichain.com/developers/multisignature-transacctions/
If neither of those are relevant, can you please explain why you need to separately sign the transaction at all?
