"I used to ... " is somewhat similar to the imperfeito in Portuguese. It means that you previously did something continuously. However, in English, "used to" also implies that you presently don't do this action:
"I used to walk to work, but now I drive my car (so, I don't walk to work anymore)."
"I am used to ..." means that you are in the habit of doing something. You are accustomed to doing something:
"I was used to eating eggs for breakfast, but in Italy I ate only yogurt."
"I'm used to going to bed early, but last night I went to bed at midnight."
Be careful not to confuse the special cases above with the normal use of the verb "to use" (see below).
Summary:
"I used to drive a car." = In the past, I often drove a car, but I don't anymore.
"I am used to driving a car" = I usually drive a car/I am accustomed to driving a car.
"Yesterday, I used a car" = Yesterday, I drove a car (simple past tense like pretérito).
I hope that helps! If you need any more help, I'd be happy to offer more examples.