Good morning is "Ohayo gozaimasu!" (Casual: ohayo!) Good afternoon is "Konnichiwa!" Try searching for these on Youtube :D You will find how to pronounce them!

How was your day ? as native Japanese say " Kinou wa dou datta ? nanika ii koto atta ? " or " dou ? choushi wa ? " or If you didn't much meet to her for few days more long time, you can say " saikin dou ? " " gennki ? " or " dou ? genki siteta ? " like that . ^-^ these phrase are only for friends. but of course no like slang. Never be rude if you say to your friends these words. If you say to same ages person with you or your friends polite words in Japanese, it might be too polite as a little sarcasm. However you are not Japanese. So the Japanese never think " you say to me a sarcasm " even if you say to Japanese too polite words for friendship. But I mean if JAPANESE say to JAPANESE polite words, it's might there is possibility become like say a sarcasm.