。
I'm busy with preparations for tomorrow.
Literal
Tomorrow [of-の] preparation [with-で] busy.
A minimal statement built on the [noun] + で + 忙しい pattern — で marks what one is busy with, not a location. 準備 'preparation' is a versatile Sino-Japanese noun; 明日の準備 'preparations for tomorrow' leaves what is being prepared implicit. No subject or copula — a casual statement that doesn't even bother with です.