I have to get up early tomorrow — we're going on a picnic.
Literal
Tomorrow [topic-は] early-rising must. Picnic [purpose-に] go [because-のだから].
Two sentences linked by のだから, which provides the explanation: 'because we're going on a picnic.' 早起きしなければならない uses the suru-verb 早起きする ('to get up early'). The のだから gives a strong explanatory force — 'the reason is that...'