Start small and start simple. Experience is your best teacher. You didn't mention any prior training or qualifications you might have. A piece of paper is not nearly as good as word-of-mouth, but it would help you attract prospective clients at first.
You should have some idea of your target market. Of course you want to attract whomever you can, but you should concentrate first on your 'easiest' students. Middle School to High School students are already focused on learning and they can give you feedback on your methods. Encourage them to ask questions from their lessons at school. This will help you tailor your program to help them most. Adults with busy professional and family lives are the worst students. When they cancel (they will- often and for good reasons), it's money out of your pocket. You want students who can keep a regular schedule.
Where will you hold your lesson meetings? If at home, then you should prepare a 'classroom'. This ties in with the number of students per 'session'. One at a time will be best for you and for your students. I wouldn't allow more than two at a time, unless you are a very experienced classroom teacher (I'm guessing you are not).
Your classroom need not be elaborate, but should make students feel like they in a classroom environment. A straight-backed chair and a small folding table are quite adequate. Older students who only need some conversation or reading practice can sit on a sofa, but younger kids need more structure. The largest whiteboard you can manage will certainly be useful. Make sure you have good lighting.
For assessment, prepare a sheet with some sample sentences containing words, pronunciations, and usages you found difficult when you were a student. Have the students read as you make notes. You will soon begin to know after a few minutes what each student's weaknesses are.
Have a handout stating to parents (or adult students) what they can expect from you, what you expect from them, and your pricing structure. Have business cards.
As to what to teach, I think you know better than I do how Portuguese and English compare/contrast. We teach in China, and older students have no time for extra lessons, so our program focusses on basic phonics with a bit of structure (verb tenses) brought in as students progress.
The most important thing I can stress is: Treat this as a business. The relationship between you and your students should be of mutual benefit. Benefit for you means more than just the money they pay. If you have a 'bad' student, let them go! Word of mouth is your best marketing tool. "I hear your daughter is taking lessons from Nohead. I'm thinking of sending my boy. How's she doing?" "Not so good- she's been going for 6 months, but she still isn't doing any better at school." Her lack of progress may well be because she doesn't want to be there or maybe she's just plain dumb, but that impacts on you either way. Far better to find a tactful way to cut off the relationship yourself.
Keep accurate records and write a receipt for money paid.
Good Luck!