How do you use legal in Brazil?

Use legal as a reaction (Que legal!, "how cool!"), about a thing ("it's cool") or about a person ("nice, fun"). It doesn't change for feminine words (uma ideia legal); the plural is legais.

PortugueseMeaningIn CiaoSpeak
legalcool, great (everyday speech); also legal (by law)Unit 1
Ah, que legal!Oh, how cool!Speak: Language exchange
Que legal! O rio aqui é enorme, viu?How cool! The river here is huge, you know?Speak: River trip
Ele é muito legal.He's really nice.Unit 13
super legalsuper cool, really nice (casual)Unit 34
Ele é legal demais!He's so nice! (casual)Unit 34

Does legal mean legal in Portuguese?

It can: legal is also the normal word for "lawful". When the law is what you mean, Brazilians often say permitido ("allowed") to be clear, because É legal? can sound like "Is it cool?".

PortugueseMeaning
É permitido?Is it allowed?
É legal?Is it cool? (and, in the right context, "Is it legal?")

What is the opposite of legal?

The opposite is chato or chata: "boring" about a thing, "annoying" about a person. To say you're not into something, Não é a minha praia ("It's not my thing").

PortugueseMeaningIn CiaoSpeak
chato / chataboring; annoyingUnit 9
Não é a minha praia.It's not my thing. (literally "not my beach")Unit 28

How else do Brazilians say cool?

Besides legal, Brazilians say demais! ("awesome!"), show! and top; in the Northeast you'll hear massa. More in the Brazilian slang list.

PortugueseMeaningIn CiaoSpeak
Demais!Awesome! (casual)Unit 34
Show!Great! Awesome!Not in the course
Ficou top!It turned out great!Not in the course
massacool, great (the Northeast and Brasília)Not in the course

How can you practice Brazilian Portuguese out loud?

Practice it in short, real exchanges, out loud. CiaoSpeak's Brazilian Portuguese course teaches everyday words like legal with a recording in a Brazilian accent, and in the Speak tab people across Brazil (a São Paulo baker, a Rio ride-app driver, your language-exchange partner Ana) ask you one question at a time. You answer by voice or typing, and the app tells you whether a Brazilian would understand you. Lessons are free; in the Speak tab, today's questions (up to five a day) are free too, and CiaoSpeak+ adds practice in any scene whenever you like. More words like this one: the Brazilian slang list.