I'm a Canadian citizen and I did two internships in the states last term. What forms do I need to fill out for the US Tax Return 2012? Is there a guide for people like me? Any help would be appreciated.
You can start with the IRS Publication 519 - US Tax Guide for Aliens. Might be slightly overwhelming.
You need to file form 1040NR (and a similar form to the State where you were working). If you're studying in a US university, it is likely that the International Students office can get you access to GLACIER (formerly known as CINTAX) - a special tax preparation program designed for international students and visiting scholars. If you're coming from a Canadian school, check with them. If they send interns to the US routinely, they might have access to it too.
You should also verify that you get the benefits you're entitled to under the US-Canada tax treaty.