You should consult with an experienced immigration attorney to discuss the eligibility for different visas. One of the big questions is whether you want to get married in Canada or the U.S., because that will determine what process would be followed. An attorney can explain the full process from start to finish once you determine which process works best for you and your girlfriend.
The answer provided does not create an attorney-client relationship and is only based on the information submitted in the question. If other information were submitted, it could result in a different answer. Consult with an experienced immigration law attorney for legal advice.
The forms are I130, I485, I131, I765, I864, and G325. Read each of the instructions carefully. Make sure you understand INA 245(a) and your burden to prove admission. Aldo read up on the caselaw and how to prove the validity of the marriage. Or you can retain counsel who is experienced in these matters and allow them to do the filings for you and attend the marriage interview with you.
Whether you need an immigration attorney depends on whether you understand the process. If you do not, then, yes, you do need an immigration attorney.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
If after reading all the forms, you still have questions -- you need an attorney to represent you. Make sure you include ALL supporting documents with your forms. Good luck!