“How are you?” means “Как дела?” (“Kak dela?”). If you ask your friend, you can say “Как ты?” (“Kak ty?”) or “Как все складывается?” (“Kak vsyo skladyvaetsya?”). “I’m fine.” means “Я в порядке.” (“Ya v poryadke.”) in Russian.
Or you can say “Все хорошо.” (“Vsyo khorosho.”) or just “Хорошо.” (“Khorosho.”), or “Замечательно!” (“Zamechatel’no!”) instead of “I’m fine!”.