Trying to write in Romanji I

Ohayo min'na! Watashi no namae wa Kallebe desu.

(I would say "minasan" because in my opinion, using formal way to say is better for the first time you introduce yourself, "minna" sounds friendly but informal way to say.) 

Nihon'nobunka ni deatta toki kara, nihongojin no benkyo wo shi hajimemashita.

(Writing "Nihon no bunka" would be better. the Japanese people=nihonjin, the Japanese language=nihongo.  hajimeta (informal), hajimemashita (formal))

Watakushihadeshou kokugo matawa eigo de tōkupātonā o mitsuketaidesu.

(I would say: Watashi wa porutogarugo to eigo wo hanasemasu. Nihongo wo oshiete kureru tōkupātonā wo sagashite imasu. = I can speak Portuguese and English. I'm looking for talk partners who can teach me Japanese. "kokugo" means "my own language", so it can be different depending on the person.) 

Sonotoki waya, yoroshiku onegaishimasuho!

Good job! I think many people will help you here in italki, please keep up and enjoy learning Japanese! 

I hope this was helpful and my English wasn't bad. Muita sorte!