"Tomorrow she is busy."
Translation:Она завтра занята.
It's the same in English: you can say "Tomorrow she will be busy", but that's not the original sentence. Using present rather than future tense when talking about future events conveys their expected or planned nature (in both languages).
That really depends on what you are trying to say. In Russian, the last word is typically the one introducing new information. Hence "Она завтра занята" is a statement about her schedule tomorrow, while "Она занята завтра" stresses "завтра" and likely implies that she is free on other days.