“obrigado” is usually an adjective applied to the person who says “thank you”. So a man should say “Obrigado.” and a woman should say “Obrigada.” But in exceptional situations "obrigado" can be an interjection. In those cases, even if you are a girl you need to say "Obrigado!"

you have to say Obrigada! Obrigado is for male

It's not completely wrong,but usually you should say just 'Obrigada' because this only applies for girls^^

You must to say "Obrigada".

You can read here some articles and learn more: http://www.italki.com/articles/portuguese