It is a kind of difficult for us native Japanese speakers to explain the distinction between 'は' and 'が' because we unconsciously distinguish them. But I will try to do it. The basic way of deciding which particle should be used is to see whether the word you are about to say is related to something particular in the topic. 'は' should be fine unless the word is related to something particular. When you introduce yourself, it would sound unnatural if you said "私がブラジル人です / Watashi ga burajiru-jin desu". It would sound like "I am THE Brazilian". People might imagine that you belong to an international group and someone asked the members of your group "Who is the Brazilian in this group?" Therefore "I am Brazilian" should be translated into "私はブラジル人です / Watashi ha burajiru-jin desu". Examples: 私はサンバダンサーです。/ Watashi ha samba dansah desu. --> I am a samba dancer. 私がサンバダンサーです。/ Watashi ga samba dansah desu. --> (Who is the samba dancer?) I am THE samba dancer (among us). 私はフェジョアーダを食べます。/ Watashi ha feijoada wo tabemasu. --> I (usually/customarily/generally) eat feijoada. 私がフェジョアーダを食べます。/ Watashi ga feijoada wo tabemasu. --> (Who will eat this feijoada?) I will eat THE feijoada. あなたは可愛い。/ Anata ha kawaii. --> You are cute. あなたが可愛い。/ Anata ga kawaii. --> (You think every girl in my class is cute, don't you?) You ARE cute. Not anyone else but you. The above is just a basic. How to use 'は' and 'が' is one of the most difficult points to acquire for Japanese learners. Even high lever learners often use an unnatural expression. So you don't have to worry about it so much. Take it easy and Just go ahead!