A person with a student visa is not supposed to have immigrant intent. That is, the student is not supposed to be intending to live permanently in the US. Having a US citizen spouse may make proving a lack of immigrant intent very difficult. Thus, if the border inspector finds out about your Brazilian marriage, you may be denied entry into the US. Border inspectors have broad discretion in who can be admitted. If you are lucky, you would be inspected by one who would let you in.
For immigration, there is no paperwork you need to fill out before the wedding. You likely will need to fill out paperwork to apply for a marriage license.
After the wedding, there are many forms you and your spouse will need to submit. The forms and instructions are free at www.uscis.gov . For details, search past posts or legal guides this site.
If you marry in the US, you likely can fill out and submit the application for travel documents. If you are currently in a valid immigration status, you likely would not have any problem in using the travel documents to leave and return to the US.
USCIS is supposedly processing paperwork much faster than it was even just a few years ago. If everything checks out for you, you may even get legal permanent residency within six months after filing the petitions.
There are many potential problems in filing immigration petition. You should review your specific facts with your attorney to find out the potential problems and how to solve them.