"Fine, thank you."
'dobry' is an adjective. It could possibly be a translation of 'fine', but 'fine' is clearly an adverb here. The adverbial form is 'dobrze'.
'dobra' is very coloquial. To a friend you could say ' dobra, dzięki' but in most cases you would say ' dobrze, dziękuję'.
I am using my Samsung phone and can't get the Polish characters done. Please help!
Settings > Language and input > Language. There you can add additional keyboard settings. Look for Polish there.