Hi Hoi,
You'll want to click on your model in the Analytics section:
>>Model Actions
>>Lead Assignment Rules.
That's the trigger that will automatically assign to the first stage you'd like.
THAT BEING SAID:
You can also do it through a smart campaign...in fact I run most of my triggers for RCM through Smart Campaigns (and batch nightly campaigns).
Lead Is Created Trigger works the same as above assignment rules.