"До свидания" (the words stay separetly!), but we pronounse it "Дас-видАнья".
"How do you do?" - "Здраствуйте". Iт fluent speach it's "Здрасьте" or even "Здрасть..."
In informal - "Зда-рОва!" (like "Зда-рОво! Ты бык'ая-карОва!")
If it's time to leave and you want to say it in informal way you may use "Ну...пакЕда!" or "Ну... бывай!" ("бывай здоров").
But I think the informal is too early for you. The main point here is a correct intonation, which I hope you'll acquire only after the fourth tick of your language scale).